Invalid hardcoded paths for data files in cache LicenseIndex

```
>>> from licensedcode.cache import *
>>> idx=get_index()
>>> rule = idx.rules_by_rid[1200]
>>> rule
Rule(identifier='agpl-3.0_371.RULE', license_expression='agpl-3.0', minimum_coverage=80, is_continuous=False, relevance=100, has_stored_relevance=True, length=6)
>>> rule.data_file
'/home/runner/work/scancode-toolkit/scancode-toolkit/src/licensedcode/data/rules/agpl-3.0_371.yml'
>>>
```

Here `'/home/runner/work/scancode-toolkit/scancode-toolkit/src/licensedcode/data/rules/agpl-3.0_371.yml' ` is the absolute path if the data files as they existed on some obscure CI runner that was used to create the index... the paths should be relative and "portable" on another machine!