Add information about update site in ModuleSearchResult

[imagejan]

When search a specific command, it might be useful to not only find its Location (i.e. the containing jar file) but also which update site is providing the jar and therefore the command.

[ctrueden]

IIRC, creating a new FilesCollection triggers a rechecksum of all the artifacts, which we wouldn't want to do every time someone does a search. The updater needs some love to cache the results of the initial update check, so that questions like "to which update site does this JAR belong?" can be answered repeatedly in a more performant way.