portz_uninstall: remove also PKGINFO which doesn't exist in MANIFEST due to error (fixed in install_staging.sh)
package_defs(package_from_portz_file): unset variables before sourcing defs; be more verbose and show what has been loaded
portz_install: sumary fixed (failed packages) and extended (successfull installs are now printed)
git loading logging fixed
check_presence_step added; so package can override <check-if-present> condition when testing dependencies
merge of '43b5f6631ccc84a27d0040c868b8e07dbaf30c74'
     and '4879c3469b6bfc7a76a4e17a9ccf659b61938be6'
portz_install: load build_defs before install, so new env is accounted after installing deps