Break up requirements.txt

[ChrisCummins]

This project currently has dependencies on 149 pip packages. In order to run any python target, all 149 dependencies must be downloaded and built. This makes the initial build of a python target take a long time and a lot of disk space, and increases the dependencies of this project by making every python target require all build dependencies for all packages.

If we could break up requirements.txt so that only the pip packages required for a build of the current target are loaded, this would greatly improve things.

[ChrisCummins]

See commit 4d6cba1

[ChrisCummins]

Related reading bazel-contrib/rules_python#96