Construct flake8 excludes for all packages in defined repos

[greenc-FNAL]

Construct a --per-file-ignores command line option for flake8 with more
lenient exclusions for packages in each defined repo.

This PR does not cause flake8 (or any other style tool) actually to
check repos not in the prefix tree, but it will apply the usual lenient
exclusions if any packages are checked as a result of non-option
arguments on the spack style command line.

This can be added to this PR if desired.

Notes:

1. The command line option overrides any per-file-ignores entry in the
   spack/.flake8 configuration file.
2. We currently use a hardwired list of exclusions in style.py. This
   can be stored in a separate file, or pulled from the main
   per-file-ignores entry in spack/.flake8 if preferred.