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

[jeherve]

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. Could we bail in those scenarios and not try to include the file at all?

[swissspidy]

I noticed that too when running unit tests against trunk in a plugin. Would be good to fix this, and the patch looks reasonable for that!

[jeherve]

Code is now also available at ​https://github.com/WordPress/wordpress-develop/pull/129 if needed to check Travis results.

[jeherve]

Ensure that we pass a full path

[gziolo]

It looks like a good enhancement if we want to allow running unit tests without having JS build files bootstrapped.

[gziolo]

In 47048:

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.