packages/pkgbuild: pkgbuild_sanitize: reinstate CLEAN_BUILD flag.

Cleaning build directories by default partially defeats purpose of the
build tree.
Also, if pkgbuild_sanitize is called right after pkgbuild_update, the
newly extracted VCS repositories are removed and then extracted in
second time (which is inefficient).
packages/pkgbuild: _pkgbuild_update_local: add missing stderr redirection.
packages/pkgbuild: add pkgbuild_fix_source_list: maintenance function to amend commits after changing/fixing pkgbuild_ls_files semantics.