packages/pkgbuild: consider also $source and $install in pkgbuild_ls_sources.
packages/pkgbuild: fix Git repository initial cloning.

- do not enquote expansion of ${_gitbranch} as it shall expand to
  emptiness, not empty parameter if ${_gitbranch} is unset
- use --depth=1 only with git-aware transport (git://)
packages/pkgbuild: fix SVN repository initial cloning.

Create source directory before descending there.
packages/pkgbuild: add CLEAN_ALL parameter to pkgbuild_sanitize.
packages/pkgbuild: initialize internal Git repository automatically on PKGBUILD version check.