Tweak README formatting

GitHub adds an awful lot of whitespace, try to work around it.
Tell dpkg-source there is no need to copy tarballs
Rework the cache layout
Check for path traversal
Continue cache rework

Now correctly validates multiple dscs for the same version.
Don't use first seen timestamps as a fallback.
Check for dscs with distincts cleartexts

Checking for distinct dscs has too many false
positives (such as curl).
Don't check signatures twice
Update depth note
Fail earlier on invalid gpg uids
Add a SrcPkg type
Move the json cache to a subdirectory
Mention the kid with an invalid uid
Avoid cache pollution with irrelevant filenames