Build tools: Combine webpack config files.

This prepares us for building the Gutenberg packages.

Merges [43687] to trunk.
See #45065.

Built from https://develop.svn.wordpress.org/trunk@43978


git-svn-id: https://core.svn.wordpress.org/trunk@43810 1a063a9b-81f0-0310-95a4-ce76da25c4cd