tensorflow: cleanup and merge with libtensorflow

This merges work done by yorickvP and timokau in NixOS#63208 and NixOS#63616 respectively.
Now the derivation builds both libtensorflow and the Python package and puts them into
different outputs.

Quite a bit of improvements were done on the top, including:

* Use official tag revision as source, not a branch;
* Use all system libraries possible (before only one was actually used);
* Move various environment variables to the derivation itself from hooks;
* Use source Python build instead of wheel build to ensure fixup hooks do their important jobs on libraries;
* And more that I forgot!