Add pants source roots and a constant for pack common lib directory

[cognifloyd]

Background

This is another part of introducing pants, as discussed in various TSC meetings.

Related PRs can be found in:

• pantsbuild milestone (which includes PRs since Initialize pantsbuild #5713)
• pantsbuild label (which also includes preparatory PRs that came before adding pantsbuild)

Overview of this PR

I ran into an issue with dependency inference not understanding the "lib" directories. ST2 adds <pack>/lib to PYTHONPATH for both actions and sensors, and <pack>/actions/lib to PYTHONPATH for actions. Pants uses "source roots" to manage the PYTHONPATH, so we need a way for pants to detect those source roots when needed. So, I adjusted the [source].root_patterns setting in pants.toml to account for these lib dirs.

While investigating this, I had to dive into the code that defines PYTHONPATH for running pack actions/sensors. I found the "lib" string to be rather confusing as only some of them referred to our special common libs directory. So to clarify that code, I added a COMMON_LIB_DIR constant to clarify which lib is which (ie to distinguish it from virtualenv/lib and virtualenv/python*/lib). I also adjusted one other place that was not using the action LIBS_DIR constant like everything else that references <pack>/actions/lib.

Pants documentation

• Source roots