Implement RFC 18: Reorganize vendor platforms

[whitequark]

RFC: https://amaranth-lang.org/rfcs/0018-reorganize-vendor-platforms.html

• Reorganize vendor platforms: Implement RFC 18: Reorganize vendor platforms #887
• Remove deprecated names for platforms: Remove names for vendor platforms that were deprecated in RFC 18 #1002

[gatecat]

Some implementation questions I have:

• how should the lazy loading work? just checking all the vendor modules to see which one the class is in, or having a class->module dict in the __init__.py? (not sure if there's a standard python paradigm here I just don't know and couldn't immediately find)
• do we need backwards compatibility?
• in amaranth-boards there's a bunch of from amaranth.vendor.lattice_ecp5 import *, this would be refactored to specific imports like from amaranth.vendor import LatticeECP5Platform?

[whitequark]

• having a class->module dict in the init.py?

I'm perfectly happy to have something like:

def __getattr__(self, name):
    if name == "XilinxPlatform":
        from ._xilinx import XilinxPlatform as platform_cls
    elif name == "...":
        ...
    return platform_cls

• do we need backwards compatibility?

Yes. Please leave modules like vendor.xilinx which just import the right class, and issue a deprecation warning. They will be removed after Amaranth 0.4 is released.

• in amaranth-boards there's a bunch of from amaranth.vendor.lattice_ecp5 import *, this would be refactored to specific imports like from amaranth.vendor import LatticeECP5Platform?

Yes