MDL-77333 mod_resource: fixes generator uploading files + tests
MDL-76499 revealed a few problems with resource generators:
1. We were not covering with unit tests the upload of files from disk
(and here it's where the problem was).
2. There was a little of confusion between disk paths (only needed
to upload files) and file_area paths (the generator only creates
or uploads files to the root directory of the file area.
3. It was possible to request the upload of a file to the generator
without that file effectively existing.
This commit fixes those points and covers 99% of the generator code.