Split out of #3266
The "selector" concept doesn't seem well enough defined as-is. For
example, `PythonVersion` belongs there but isn't present. Going for
smaller modules instead.
Split out of https://github.com/astral-sh/uv/pull/3266
If `UV_BOOTSTRAP_DIR` and `CARGO_MANIFEST_DIR` are both unset, we
currently panic. This isn't good once we start to use managed toolchains
in production. We'll need to change this more later once the toolchain
directory is more user-facing.
Moves all of `uv-toolchain` into `uv-interpreter`. We may split these
out in the future, but the refactoring I want to do for interpreter
discovery is easier if I don't have to deal with entanglement. Includes
some restructuring of `uv-interpreter`.
Part of #2386
renovate[bot]