refactor: add builders to make creating PyInfo/depset/runfiles easier

[rickeylev]

This add some builders "classes" to make constructing PyInfo, depsets, and runfiles
a bit cleaner. Previously, building such objects required creating multiple local
variables for each arg/attribute of the object (e.g. list_of_files and list_of_depsets
for building a depset). Keeping track of all the variables was verbose and tedious.

To make it easier, encapsulate that bookkeeping in some objects. Accumulating parts is
now much easier: builders have an add() method that looks at the input and stores it
in the appropriate place.

This builders will be exposed to users in an upcoming PR to make building and merging
PyInfo objects easier.

Work towards #1647