Migrate DefinitionMap logic to its own class.

This doesn't change any functionality. It just compartmentalizes a large
chunk of logic that was previously intertwined in the Compiler class.
This logic stands alone and doesn't need anything from the Compiler at
all except the generic "Defined" expression from the Context.

In followup CLs I intend to experiment with changes to the API and
logic, but factoring the code out seemed like a big enough change that
it deserved its own CL.

Change-Id: I3e1a7c62812c6f284167c967086ef4dd828a0b2e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367879
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed