resourceloader: Support 'versionCallback' for computed package files

resourceloader: Support 'versionCallback' for computed package files

The use cases we've seen for using computed (or virtual) package files,
involve expensive computations to expand and transform data for the client
that we don't want to evaluate in full just to compute the module's version
hash (e.g. in the StartupModule, where we need to do this for 1000s of
modules).

For such cases, the module can specify a 'versionCallback' of which the
return value will be used to seed the module's version hash. The default
remains the same as before, which is to use the full content to seed the
version hash (via getDefinitionSummary).

Bug: T223260
Change-Id: I76f573239e6bd429287e7adb33a92ffd5e260c20