Prefer Python executable names that match the request over default names (#9066)

This restores behavior previously removed in
https://github.com/astral-sh/uv/pull/7649.

I thought it'd be clearer (and simpler) to have a consistent Python
executable name ordering. However, we've seen some cases where this can
be surprising and, in combination with #8481, can result in incorrect
behavior. For example, see https://github.com/astral-sh/uv/issues/9046
where we prefer `python3` over `python3.12` in the same directory even
though `python3.12` was requested. While `python3` and `python3.12` both
point to valid Python 3.12 environments there, the expectation is that
when `python3.12` is requested that the `python3.12` executable is
preferred. This expectation may be less obvious if the user requests
`python@3.12`, but uv does not distinguish between these request forms.
Similarly, this may be surprising as by default uv prefers `python` over
`python3` but when requesting `python3.12` the preference will be
swapped.

crates/uv-python/src/implementation.rs

@@ -33,6 +33,10 @@ impl ImplementationName {
         ["cpython", "pypy", "graalpy"].into_iter()
     }
 
+    pub(crate) fn iter_all() -> impl Iterator<Item = Self> {
+        [Self::CPython, Self::PyPy, Self::GraalPy].into_iter()
+    }
+
     pub fn pretty(self) -> &'static str {
         match self {
             Self::CPython => "CPython",