clean up imports & move exports to dedicated file

This slightly reorganizes the import forms for git & git-lolevel, moves
git's export list into its own file, and adds an explicit export list
for git-lolevel (rather than just exporting everything), also in its own
file.
check pkg-config for libgit2 version information

This checks pkg-config for libgit2's version information before
installation (and exits early on a version mismatch), if it's available.
produce object files and an archive for static linking
fall back to repo-discover when initial repo-open fails