pythonlib: improved dependency management and examples

[jodersky]

Make the dependency management more flexible:

• allow users to re-use requirements.txt files (inspired by the same pants feature)
• separate library and tool requirements
  • library deps are transitive, and will in the future be included as requirements in distributions
  • tool requirements are not transitive and won't be included in distributions. They're used for things like type checkers and bundlers (which can all be extracted into separated traits later on if we want to).
• support referencing wheel files directly
• allow user-defined PYTHONPATH
• custom package repos
• lockfile support Removing this from this pull request, since it's likely a bit more complex than I initially thought. Pants delegates this behavior to PEX, but we should see if this can be done with more basic tools, or we decide to rely on pex for everything.
• examples for all the above

Part of #3928

[himanshumahajan138]

@jodersky Sir I have done the Basic example and if you want/allow i can help in writing examples for this also...

[jodersky]

thanks, however I already started working on the examples

[lihaoyi]

Looks great