(PUP-1892) File resources do not work with fileservers that are not the current puppetmaster.

the source parameter makes the 'uri' method private; assumably for security, the actual result is that
noone can get the full URI from the source param. The end result is that when collecting the source data
the file_content class uses full_path instead, which gets magically completed to use the puppetmaster.

If your fileserver is in fact some other remote host, that data is lost. This change makes 'uri' public
and adjusts the content handler so that it properly delivers either a URI as a string, or a
full path without leading slash (which is what the previous state was.)