feat: add public API for analysis-phase logic

[rickeylev]

This adds a public API for rules (i.e. analysis-phase code) to use code from rules_python.
The main motivation for this is so that users can propagate PyInfo without having to know
all the fields of PyInfo and implement the merging logic. With upcoming PRs adding additional
fields to PyInfo, this becomes much more important.

The way the API is exposed is through a target. There are three reasons for this:

1. It avoids loading phase costs when the implementation of the API functions change.
   Within Google, this makes changes to rules_python much cheaper and easier to submit
   and revert. This also allows us to worry less about the loading-phase impact of
   our code.
2. Because a target can have dependencies, it allows us to hide some details
   from users. For example, if we want a flag to affect behavior, we can add it to the
   API target's attributes; users don't have to add it to their rule's attributes
3. By having the API take the user's ctx as an argument, it allows us to capture it
   and use it as part of future API calls (this isn't used now, but gives us
   flexibility in the future).

Work towards #1647

[aignas]

LGTM