extend archive index with lists, use in source view for tree

[syphar]

• right now we're storing a big JSON structure in releases.files, containing a list of all files in the source archive.
• It's used to generate the file tree in our source handlers.
• But: we already have the file list available in the archive index that belongs to the source archive, so we can use that and be much more efficient.

So this PR:

• updates the storage lib to provide access to file lists in the index, and prefix filters
• the source handlers now use the index, if archive storage is true.

Next steps after this:

• perhaps stop storing the file list for new builds / releases
• stop passing around the file-lists in results from storage, builders, ...
• or, if repackaging is done, directly remove the column when we remove the legacy storage.

[GuillaumeGomez]

Looks good to me, thanks!