Consider taking into account imported scripts via `importScript` to generate cache name

[delapuente]

This is needed in case scripts imported via importScript change but not the assets. A current workaround is to include imports as assets. A more abstract, generic way is to allow the inclusion of a version mark in the service worker provided by the consumer to force service worker update.

[mykmelez]

Hmm, good point! Perhaps getHashOfHashes could hash the import scripts and include those hashes in its hash of hashes, so the cache version changes when import scripts change, even if import scripts aren't explicitly offlined.