Subcommand lint was looking for srpm only in repository root,
and for rpms only with globs of form %{ARCH}/*.rpm.
This works in many cases,
but not when the config option results_dir is set to subdir
or when (if ever) SRPMLayout is used.
Fixed by using relevant layout parameters
when looking for files to lint.

It is difficult to account for layout parameter rpmfilename properly.
In addition to a basename,
that parameter can contain a path relative to rpmdir
and may contain rpm macros.
Since macro evaluation is not possible in the rpkg lint context,
a simple heuristic is used instead,
covering the known cases:

1. only basename
2. relative path %{ARCH}.

Also, support for linting mockbuild results is added.
As mockbuild is arguably even more important than local build,
cases where both mockbuild and local build results are present,
mockbuild results are selected.

Resolves #586