[release-1.6] Add manifest julia_version difference warning

[IanButterworth]

As discussed in #2665 I think it would be helpful for 1.6.2 to have something like this. Otherwise the user will get unactionable errors with no warning.

This is a 1.6-appropriate modification of the warning that was omitted in #2620 when backported in #2628

[KristofferC]

I think this is a bit too late to go into 1.6.2 but could go into 1.6.3.

[IanButterworth]

Fair

[DilumAluthge]

Is this good to go? So we can get it into the next 1.6.x?

[DilumAluthge]

Can we get this into 1.6.6?

[IanButterworth]

I think that's a good idea. Just added some tests.

[IanButterworth]

To illustrate with this Manifest.toml

1.6.5

(@v1.6) pkg> activate ~/.julia/environments/v1.7
  Activating environment at `~/.julia/environments/v1.7/Project.toml`

(@v1.7) pkg> instantiate
ERROR: AssertionError: sourcepath !== nothing
Stacktrace:
  [1] is_package_downloaded(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
    @ Pkg.Operations ~/Documents/GitHub/Pkg.jl/src/Operations.jl:1789
  [2] #13
    @ ~/Documents/GitHub/Pkg.jl/src/Operations.jl:131 [inlined]
  [3] _all
    @ ./reduce.jl:923 [inlined]
  [4] #all#698
    @ ./reducedim.jl:886 [inlined]
  [5] all
    @ ./reducedim.jl:886 [inlined]
  [6] is_instantiated(ctx::Pkg.Types.Context)
    @ Pkg.Operations ~/Documents/GitHub/Pkg.jl/src/Operations.jl:131
  [7] instantiate(ctx::Pkg.Types.Context; manifest::Nothing, update_registry::Bool, verbose::Bool, platform::Base.BinaryPlatforms.Platform, allow_build::Bool, allow_autoprecomp::Bool, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API ~/Documents/GitHub/Pkg.jl/src/API.jl:1360
  [8] instantiate
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:1325 [inlined]
  [9] #instantiate#252
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:1321 [inlined]
 [10] instantiate()
    @ Pkg.API ~/Documents/GitHub/Pkg.jl/src/API.jl:1321
 [11] do_cmd!(command::Pkg.REPLMode.Command, repl::REPL.LineEditREPL)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:405
 [12] do_cmd(repl::REPL.LineEditREPL, input::String; do_rethrow::Bool)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:386
 [13] do_cmd
    @ ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:377 [inlined]
 [14] (::Pkg.REPLMode.var"#24#27"{REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:550
 [15] #invokelatest#2
    @ ./essentials.jl:708 [inlined]
 [16] invokelatest
    @ ./essentials.jl:706 [inlined]
 [17] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/LineEdit.jl:2441
 [18] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:1126
 [19] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL ./task.jl:411

This PR on 1.6.5

(@v1.6) pkg> activate ~/.julia/environments/v1.7
  Activating environment at `~/.julia/environments/v1.7/Project.toml`

(@v1.7) pkg> instantiate
┌ Warning: The active manifest file has dependencies that were resolved with a different julia version (1.7.0). Unexpected behavior may occur.
└ @ ~/.julia/environments/v1.7/Manifest.toml:0
ERROR: AssertionError: sourcepath !== nothing
Stacktrace:
  [1] is_package_downloaded(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
    @ Pkg.Operations ~/Documents/GitHub/Pkg.jl/src/Operations.jl:1789
  [2] #13
    @ ~/Documents/GitHub/Pkg.jl/src/Operations.jl:131 [inlined]
  [3] _all
    @ ./reduce.jl:923 [inlined]
  [4] #all#698
    @ ./reducedim.jl:886 [inlined]
  [5] all
    @ ./reducedim.jl:886 [inlined]
  [6] is_instantiated(ctx::Pkg.Types.Context)
    @ Pkg.Operations ~/Documents/GitHub/Pkg.jl/src/Operations.jl:131
  [7] instantiate(ctx::Pkg.Types.Context; manifest::Nothing, update_registry::Bool, verbose::Bool, platform::Base.BinaryPlatforms.Platform, allow_build::Bool, allow_autoprecomp::Bool, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API ~/Documents/GitHub/Pkg.jl/src/API.jl:1360
  [8] instantiate
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:1325 [inlined]
  [9] #instantiate#252
    @ ~/Documents/GitHub/Pkg.jl/src/API.jl:1321 [inlined]
 [10] instantiate()
    @ Pkg.API ~/Documents/GitHub/Pkg.jl/src/API.jl:1321
 [11] do_cmd!(command::Pkg.REPLMode.Command, repl::REPL.LineEditREPL)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:405
 [12] do_cmd(repl::REPL.LineEditREPL, input::String; do_rethrow::Bool)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:386
 [13] do_cmd
    @ ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:377 [inlined]
 [14] (::Pkg.REPLMode.var"#24#27"{REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool)
    @ Pkg.REPLMode ~/Documents/GitHub/Pkg.jl/src/REPLMode/REPLMode.jl:550
 [15] #invokelatest#2
    @ ./essentials.jl:708 [inlined]
 [16] invokelatest
    @ ./essentials.jl:706 [inlined]
 [17] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/LineEdit.jl:2441
 [18] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.6/REPL/src/REPL.jl:1126
 [19] (::REPL.var"#44#49"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL ./task.jl:411