refactor: optimize venv building for namespace packages

[rickeylev]

When implicit namespace packages are used, it's common for multiple distributions
to install into the same directory, triggering the expensive conflict merging
logic. This can be observed wit our doc builds, where sphinxcontrib is a
namespace package that 7 distributions install into.

To fix, treat top-level directories that have an importable name and don't have an
__init__ looking file as implicit namespace packages and mark them as disallowed
from being directly linked. The importable name check is to exclude dist-info
directories.

[rickeylev]

Ah drat. The regex_match function is in Bazel 7, but not exposed. Well, no biggie, we can just check for . and - if that function isn't present; should work well enough.

[rickeylev]

There we go, fixed.

[rickeylev]

Can't merge this yet; need to check on a bug it might introduced where top-level files that are merged via conflicts aren't being linked correct anymore i.e. nvidia/__init__.py is being created as a directory that contains all conflicting entries.

[rickeylev]

Well, there is a bug, but its not due to this PR. I'm not sure if its pre-existing or was introduced by #3448.

In any case, right now, the conflict merge logic won't produce the correct result if the same file is provided twice. It treats all the entries as directories, so tries to group the conflicting files under a directory named after the file. In practice, this would mostly only affect pkgutil namespace packages with non-trivial logic in __init__.py; it's uncommon for different distributions to overlap with other files.

[rickeylev]

Sent #3458 to fix that bug

[rickeylev]

dependent PR merged, PTAL