[rpkg] A new format for 'sources' files

[Mathieu Bridon]

As part of the move away from MD5 for the source files hashes in the lookaside
cache, we have decided to change the format of the 'sources' files, so it
includes the hash type, to make it easier to identify which hash was used.

We agreed on using the BSD-style format, as returned on Linux by passing the
extra --tag option to commands like md5sum, sha512sumn, etc.

In addition, we decided to forbid mixing hash types in a single sources file,
forcing maintainers to use `fedpkg new-sources` the first time they upload an
additional source file.

This 4 patches series implements all that, with loads of unit tests, and some
code cleanup.

 src/pyrpkg/__init__.py |  60 ++++-----
 src/pyrpkg/sources.py  | 123 ++++++++++++------
 test/test_sources.py   | 334 ++++++++++++++++++++++++++++++++++--------------
 3 files changed, 359 insertions(+), 158 deletions(-)

https://fedorahosted.org/rel-eng/ticket/5846