Rename the build files (libgit2#724)

This change renames the build files so they come lexicographically
before any source files. This makes the compile errors (due to
mismatched libgit2 versions) easier to understand, since the
`Build_*.go` files will be tried before the rest, and the `#error` in
those files will kick in, leading to a much better experience.

This unfortunately goes a bit against the defacto standard of using only
lowercase characters in filenames, but the better developer experience
(and better self-diagnosis when things go wrong instead of having to
open a new issue) is worth the deviation.

Fixes: libgit2#711
Fixes: libgit2#617

Author: lhchavez