Hide implementation details of `amaranth.hdl`

[whitequark]

I propose:

• [x] documenting the complete set of names under amaranth.hdl that are provided for use by Amaranth programmers #785
• [x] making all of the existing modules under amaranth.hdl private (starting with _);
• [x] adding a compatibility shim for accesses through the currently public modules;
• [x] 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.

[whitequark]

Reopening and moving to 0.6 since that's when the compatibility shims will be removed.