Cleanup packaging matching VCS overhaul

I'm sorry @wget but some aspects of your packaging are just too
complicated & brittle. In cleaning up the -git package I reorganized
a number of things such that restarting the build process wouldn't force
a clean build (you can use `makepkg -C` for the times you want a clean
build) and could be restarted an various other points. This makes it
a lot easier to update and figure out what Mattermost has changed
between releases.

I also formatted a lot of the code in a more Arch PKGBUILD standard way
(the way Arch tooling auto-formats arrays, etc.).

Please don't revert these changes back because one thing that's now
possible is sharing patches between the -git package and this one.

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