This is broken off of my prototype repository, and there are a handful of TODOs left to resolve.

Elaborate on why the TODO to rewrite {pip,whl}.sh as Python is harder than it looks.

This migrates the logic from pip.sh into piptool.py, which should improve portability by removing the bash dependency.

This also has the beginnings of wrapping piptool as a closed redistributable that doesn't rely on a system-installed copy of PIP, but instead uses these rules to pull pip into a PAR bundle.  Besides needing to work out the details of releasing and redistributing the PAR, we have two unresolved issues:
 * When bundled as a PAR (vs. py_binary), piptool seems to pick up the system-installed version of pip.
 * When bundled as a PAR, piptool sometimes sees cert issues resolving requirements (similar to what we see with httplib2).

With this change I am able to build/test my PR on my macbook without pip installed.  The only additional change I have locally is to switch from running piptool.py as a simple .py file to downloading/using a PAR built from this change.

I believe we still have the problem that the .par picks the host's version of pip instead of our embedded copy, but I haven't reverified that issue still exists (this just does nothing to address that issue).

move python tools under a top-level rules_python package

simplify version_test

`whl_library` rules generated by `pip_import` are now named as:
```
   pypi__{distribution}_{version}
```

Substituting illegal characters (e.g. `-`, `.`) with underscores.

Add a script for updating the tools and document this and the docs script.

This was my original name for update_tools.sh, I'd meant to remove it.