gh-59110: Extract function for _add_implicit_dirs.

[jaraco]

• Issue: zipimport needs to support namespace packages when no 'directory' entry exists #59110

[serhiy-storchaka]

I do not see benefits, Actually, this looks like pessimization. The current code stops adding implicit parent dirs if it found already added dir. Your code continues to generate them. For example, if there are foo/bar/baz/a.py, foo/bar/baz/b.py and foo/bar/baz/c.py, it will calculate foo/bar/ and foo/ three times. You need to use dict.fromkeys() to de-duplicate them.