Pystandalone should pack the most specific submodule for the entry point

[Schamper]

Currently pystandalone will try to package the root module of the entry point. It is excessive in the case of large namespace packages like dissect.

Pystandalone should try to package the most specific available package for the entry point, not the root package.

Example: if the entry point is dissect.target.tools.shell:main, only dissect.target should be packed and not all dissect packages.