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.