Build Tools: Minify the package CSS when building.

CSS coming from the `@wordpress` packages wasn't being minified in the build. This had the noteable side effect of causing RTL CSS to not be loaded when `SCRIPT_DEBUG` was set to `false`.

Props websupporter.
Merges [43905] to trunk.
Fixes #45330.
Built from https://develop.svn.wordpress.org/trunk@44252


git-svn-id: https://core.svn.wordpress.org/trunk@44082 1a063a9b-81f0-0310-95a4-ce76da25c4cd