Run StrictDepsPass in SoyHeaderCompiler

It would be great if `SoyHeaderCompiler` would run `StrictDepsPass`, so we can, e.g., fail the Build already at `soy_library` level and not wait until running the language generators.

Bonus points if it's possible to select between `--strict_deps={OFF,WARN,ERROR}`.

Start at SoyHeaderCompiler and trace how StrictDepsPass is currently run by the language generators. Check how soy_library builds are validated and how command-line options are handled before assessing the proposed --strict_deps={OFF,WARN,ERROR} behavior. Done means strict dependency failures can occur at the soy_library stage, with the requested selectable severity if feasible.

Written by the indexing model from the issue text.