Add support for exclusion lists provided in the source directory
If a file called ".craft.exclude" is found in the top-level source directory it's now automatically added to the list of exclusion files.
Providing exclusion lists in the source code has multiple advantages:
- exclusion lists are branched/versioned together with the application
- changes to the exclusion list can be tested in MRs with the actual packages
Supersedes craft-blueprints-kde!753 (closed).