NEXUS-5661: Proxy repo prefix file does not include paths only available...

... from local storage

Proxy prefixes.txt file is now always made up with merging the remotely got prefixes.txt (that might be remotely published prefixes.txt or scrape result) and _locally got_ prefixes.txt to ensure that the union (or remotely published and locally available) files will be present in published and used prefixes.txt (as nexus serves happily files from cache even if they were removed from remote repository).

Old UT PrefixFileForProxiesAreUnmodifiedTest "refurbished" into FileContainsMergedLocalAndRemotePrefixesTest as the old was testing an original requirement of auto routing, that is from now untrue (was true, but was overruled by issue NEXUS-5661).