Filter discovered Python interpreters by system preference (#3739)
Previously, we enforced `SystemPython` outside of the interpreter
discovery exclusively with source selection. Now, we perform additional
filtering of interpreters depending on if they are a virtual
environment. This should not change any existing behavior, but will make
it much easier to have consistent behavior in ambiguous cases like
https://github.com/astral-sh/uv/pull/3736#discussion_r1610072262 where a
source could provide either a system interpreter or virtual environment
interpreter.
