Support namespace packages where spec.submodule_earch_locations is a _NamespacePath object instead of a simple list and add support for cases where the layout of the source project does not follow the same structure as the package tree - meaning some custom magic might be involved in making editable imports work as intended.

Like in !31 (merged) and !32 (merged), we'll merge this so it is marked as merged, then rewrite history since 3.0.1 to immediately apply fixups. This has already been done in parallel branch rebase/fix-develop-namespace-packages.