Migrate to XDG and Linux strategy for macOS directories (#5806)

This PR moves us to the Linux strategy for our global directories on
macOS. We both feel on the team _and_ have received feedback (in Issues
and Polls) that the `Application Support` directories are more intended
for GUIs, and CLI tools are correct to respect the XDG variables and use
the same directory paths on Linux and macOS.

Namely, we now use:

- `/Users/crmarsh/.local/share/uv/tools` (for tools)
- `/Users/crmarsh/.local/share/uv/python` (for Pythons)
- `/Users/crmarsh/.cache/uv` (for the cache)

The strategy is such that if the `/Users/crmarsh/Library/Application
Support/uv` already exists, we keep using it -- same goes for
`/Users/crmarsh/Library/Caches/uv`, so **it's entirely backwards
compatible**.

If you want to force a migration to the new schema, you can run:

- `uv cache clean`
- `uv tool uninstall --all`
- `uv python uninstall --all`

Which will clean up the macOS-specific directories, paving the way for
the above paths. In other words, once you run those commands, subsequent
`uv` operations will automatically use the `~/.cache` and `~/.local`
variants.

Closes https://github.com/astral-sh/uv/issues/4411.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

Cargo.lock

@@ -1185,6 +1185,17 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "etcetera"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "136d1b5283a1ab77bd9257427ffd09d8667ced0570b6f938942bc7568ed5b943"
+dependencies = [
+ "cfg-if",
+ "home",
+ "windows-sys 0.48.0",
+]
+
 [[package]]
 name = "event-listener"
 version = "5.3.1"
@@ -4624,6 +4635,7 @@ dependencies = [
  "clap",
  "directories",
  "distribution-types",
+ "etcetera",
  "fs-err",
  "nanoid",
  "pypi-types",
@@ -5184,6 +5196,7 @@ name = "uv-state"
 version = "0.0.1"
 dependencies = [
  "directories",
+ "etcetera",
  "fs-err",
  "tempfile",
 ]