source: Switch to xz for packages and tools where possible

* Change git packages to xz
* Update mirror checksums in packages where they are used
* Change a few source tarballs to xz if available upstream
* Remove unused lines in packages we're touching, requested by jow- and blogic
* We're relying more on xz-utils so add official mirror as primary source, master site as secondary.
* Add SHA256 checksums to multiple git tarball packages

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>