This makes the code cleaner, and also documents the fact that effective_rules
recurses into imports.
No we're not adding the imported stylesheets twice, and we share code with the
invalidation analysis.
MozReview-Commit-ID: DOF2AViTlmR
Apparently my whole conception of how the list of sheets in the styleset looked
like was flawled, and we only ever get one append_sheet for the topmost
stylesheet, instead of one for each.
MozReview-Commit-ID: FMClygMJkTc
