Build Tools: do not include assets for JS dependencies that do not exist.

This is a follow-up for #48154, where we started including *.asset.php files to declare package dependencies.
This works well but creates warnings in environments where the package does not exist because WordPress was not fully built.

Props jeherve, swissspidy.

Fixes #49144.


Built from https://develop.svn.wordpress.org/trunk@47048


git-svn-id: https://core.svn.wordpress.org/trunk@46848 1a063a9b-81f0-0310-95a4-ce76da25c4cd