NEXUS-5807: Add magic bytes to prefixes file

And handle legacy files too. This will require and mean:

a) prefix file will not be loaded if it does not have the "magic"
header (either new one or the old one, which was basically the 1st line
comment generated by Nexus)
b) Central prefixes file should have "## repository-prefixes/2.0 as first line
(or whatever magic we agree on)
c) we should advertise this to anyone who is manually crafting
the prefixes file

Also, code reworked to use Guava's Closer.