Update toolchain resolution to a "best match" model

Currently, toolchain resolution selects the first toolchain that meets requirements. Several users have proposed instead using a "best match" strategy where the number of matching constraints are counted and used to select the toolchain.

This is an interesting idea and needs a lot more thought and design before it can be implemented. This issue serves as a note that we are considering the idea and a place to collect requirements.

The issue names no files, tests, or entry points. Start by reviewing the existing toolchain resolution behavior and collecting requirements for counting matching constraints; done requires a settled design before implementation can begin.

Written by the indexing model from the issue text.