libgpkg: add stable; remove fixes; update to match new style guide
libgpkg: add stable; remove fixes; update to match new style guide

- See: https://bitbucket.org/luciad/libgpkg/issue/2/please-tag-repo-or-offer-versioned

OSGeo/homebrew-osgeo4mac,OSGeo/homebrew-osgeo4mac,OSGeo/homebrew-osgeo4mac