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
