Several issues when installing packages via pip

@opacam

When defining requirements for p4a which get installed via pip, there are several issues:

If requirements list gets modified and p4a re-runs, packages containing namespace packages (https://packaging.python.org/guides/packaging-namespace-packages/) are not installed properly (although they are if deleting .buildozer folder and build from scratch).
Package install dependencies are ignored. They get installed in the local python, but are never synced to .buildozer/android/platform/build/build/python-installs/APPNAME/ unless defined explicitely. Some kind of pip freeze call before syncing or similar might be needed in order to collect dependencies.
Package sync to .buildozer/android/platform/build/build/python-installs/APPNAME/ should happen at each p4a run to ensure sane python package structure if anything changes.
zope and zope_interface recipes are obsolete. zope.interface installation works properly via pip (i think since @opacam refactoring). The fact that namespace packages contain no __init__.py is perfectly fine, the relevant information lives in related *-nspkg.pth files (e.g. zope.interface-4.6.0-py3.5-nspkg.pth) which get copied properly.

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