CLI: Validate asset names' uniqueness

[fleveillee]

All components have an asset folder, which might contain any asset files that will be copied as they are in the theme's assets folder.

Current behaviour:

We disregard the possibility that some might have conflicting names.

• If two components have the same asset file name
  • Both files are copied to the theme's assets folder; the last one copied overwrites the preceding ones.
• If a component has a JS asset file named exactly the same as a shared JS asset file
  • The component's JS file will overwrite the shared asset file of the same name.

Expected behaviour:

We stop the build and output an error when two identical asset file names were found.

• If two components have the same asset file name:
  • the build should fail
  • We should explicitly mention to the user that two asset files can not bear the same name.
• If a component has a JS asset file named exactly the same as a shared JS asset file:
  • the build should fail
  • We should explicitly mention to the user that two asset files can not bear the same name.