Help implementing strict dependency checking for Sass binary/libraries

This would greatly improve code-health in components. Currently lots of targets depend on transitive dependenices, making refactorings pretty painful. It's also easy to regress with deps due to no strict checking.

Start by locating the Sass binary and library targets and reviewing how their dependency declarations are resolved. Define done as strict checking that rejects targets relying on undeclared transitive dependencies, with coverage for dependency regressions; the issue names no specific files or tests.

Written by the indexing model from the issue text.