Hide implementation details of `amaranth.hdl`

[whitequark]

I propose:

• documenting the complete set of names under amaranth.hdl that are provided for use by Amaranth programmers Document the Python APIs in amaranth.hdl #785
• making all of the existing modules under amaranth.hdl private (starting with _);
• adding a compatibility shim for accesses through the currently public modules;
• deprecating such accesses;
• removing the shim in version 0.6

[whitequark]

We've discussed this RFC on the 2023-05-15 weekly meeting. The disposition was to merge the PR implementing the proposed changes.