Rewrite Python interpreter discovery (#3266)

Updates our Python interpreter discovery to conform to the rules
described in #2386, please see that issue for a full description of the
behavior. Briefly, we now will search for interpreters that satisfy a
requested version without stopping at the first Python executable.
Additionally, if retrieving information about an interpreter fails we
will continue to search for a working interpreter. We also add the
plumbing necessary to request Python implementations other than CPython,
though we do not add support for other implementations at this time.

A major internal goal of this work is to prepare for user-facing managed
toolchains i.e. fetching a requested version during `uv run`. These APIs
are not introduced, but there is some managed toolchain handling as
required for our test suite.

Some noteworthy implementation changes:

- The `uv_interpreter::find_python` module has been removed in favor of
a `uv_interpreter::discovery` module.
- There are new types to help structure interpreter requests and track
sources
- Executable discovery is implemented as a big lazy iterator and is a
central authority for source precedence
- `uv_interpreter::Error` variants were split into scoped types in each
module
- There's much more unit test coverage, but not for Windows yet

Remaining work:

- [x] Write new test cases
- [x] Determine correct behavior around executables in the current
directory
- _Future_: Combine `PythonVersion` and `VersionRequest`
- _Future_: Consider splitting `ManagedToolchain` into local and remote
variants
- _Future_: Add Windows unit test coverage
- _Future_: Explore behavior around implementation precedence (i.e.
CPython over PyPy)

Refactors split into:

- #3329 
- #3330 
- #3331
- #3332

Closes #2386

.github/workflows/ci.yml

@@ -144,6 +144,7 @@ jobs:
 
       - name: "Cargo test"
         run: |
+          export UV_BOOTSTRAP_DIR="$(pwd)/bin"
           cargo nextest run \
             --workspace \
             --status-level skip --failure-output immediate-final --no-fail-fast -j 12 --final-status-level slow