makeconf.py: Adds disable and enable functionality

To allow users to momentarily allow portage to "forget" an overlay
dir, the ability to disable and then re-enable an overlay has been
added.

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
reposconf.py: Adds disable and enable functionality

To allow users to momentarily allow portage to "forget" an
overlay dir, the ability to disable and then re-enable an overlay
has been added.

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
repoconfmanager.py: Adds disable and enable functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: 512316
db.py: Implements disable and enable functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
api.py: Adds disable and enable repo functions

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
cli.py: Adds disable and enable cli functions

argsparser.py: Adds --disable and --enable cli args.

X-Gentoo-Bug: 512316
X-Gentoo-Bug-URL: https://bugs.gentoo.org/512316
Centralizes common archive overlay code

This commit brings archive overlay code to one common place, in
an attempt to make maintaining and adding archive overlay types
to layman much easier.

config.py: Alters the clean_tar option to clean_archive.

tar.py: Removes doctest tests from tar.py as there is already an
existing external.py test.
Migrates run_command to utily.py
Adds layman-mounter utility script

api.py: Adds Mounter() initialization in the LaymanAPI.
constants.py: Adds MOUNT_TYPES to constants to keep track of
mountable overlay classes.
archive.py: Adds check to unmount overlays if they are being
synced.
Adds Squashfs overlay support

constants.py: Adds Squashfs overlay type to MOUNT_TYPES constants,
as well as different possible file extensions to FILE_EXTENSIONS
constants.
Adds Squashfs overlay external tests
module.py: Adds modular plugin system
Adds module directory for overlay types
bzr.py: Converts to plug-in module
cvs.py: Converts to plug-in module
darcs.py: Converts to plug-in module
g_common.py: Converts to plug-in module
g_sorcery.py: Converts to plug-in module
git.py: Converts to plug-in module
mercurial.py: Converts to plug-in module