add error that explains PackageSpec.repo is a private field

Author: IanButterworth

src/API.jl

@@ -1557,7 +1557,10 @@ end
 
 function handle_package_input!(pkg::PackageSpec)
     if pkg.path !== nothing && pkg.url !== nothing
-        pkgerror("`path` and `url` are conflicting specifications")
+        pkgerror("PackageSpec fields `path` and `url` both set, resulting in conflicting specifications")
+    end
+    if pkg.repo.source !== nothing || pkg.repo.rev !== nothing || pkg.repo.subdir !== nothing
+        pkgerror("The PackageSpec field `repo` is private and should not be set directly")
     end
     pkg.repo = Types.GitRepo(rev = pkg.rev, source = pkg.url !== nothing ? pkg.url : pkg.path,
                          subdir = pkg.subdir)

src/Types.jl

@@ -92,7 +92,7 @@ mutable struct PackageSpec
     uuid::Union{Nothing,UUID}
     version::Union{Nothing,VersionTypes,String}
     tree_hash::Union{Nothing,SHA1}
-    repo::GitRepo
+    repo::GitRepo # private
     path::Union{Nothing,String}
     pinned::Bool
     # used for input only