Allow discovering virtual environments from the first interpreter found on the PATH

[zanieb]

Closes #11214

Special-cases the first Python executable we find on the PATH, allowing it to be considered during searches for virtual environments.

For some context, there are two stages to Python interpreter discovery

1. We find possible Python executables in various sources
2. We query the executables to determine canonical metadata about the interpreter

We can't really be "sure" if an executable is a complaint virtual environment during (1), we need to query the interpreter first. This means that if you're only allowed to installed into virtual environments, we'll query every interpreter on your PATH. This is not performant, and causes confusion for users. Notably, I recently improved error messaging when we can't find any valid interpreters, by showing the error message we encounter while querying an interpreter (if any). However, this is problematic when there's an error for an interpreter that is not relevant to your search. In #11143, I added filtering to avoid querying additional interpreters, but that regressed some user experiences where they were relying on us finding implicitly active virtual environments via the PATH.

[zanieb]

This test case regresses, undoing the work from #11143 if the interpreter is at the front of the PATH. We don't see the regression in the diff because I added a new test case below which retains the previous error message, i.e., we do the right thing if the broken interpreter is not at the front of the PATH. We can do better, but the regression this pull requests fixes seems more important to address?

[zanieb]

Looking back at #2109 and #4009, it seems likely that #11143 regressed behavior there. This sort of fixes that? It's basically a reimplementation of #4032

[zanieb]

An alternative approach would be something like 1467172 which.. could also have problems, i.e., exclude interpreters incorrectly and requires querying more directories on the PATH when looking for virtual environments.