clear available pkgs cache when pkg db changes

Since repositories can be added in packages (indeed, this is the method
by which EPEL is recommended to be installed), this commit clears the
__context__ variable in which this information is stored, forcing it to
be recreated on the next call to check_db()
clear available pkgs cache when pkgrepo state has changes

This clears __context__['pkg._avail'] when a pkgrepo state makes
changes, as this changes the available packages.
Merge pull request #11025 from terminalmage/11001

Clear available pkgs cache when available packages are likely to have changed