don't use relative paths in pkg_resources

From desispect.test.test_extract:
```
/Users/sbailey/anaconda3/envs/desitest/lib/python3.7/site-packages/pkg_resources/__init__.py:1145: DeprecationWarning: Use of .. or absolute path in a resource path is not allowed and will raise exceptions in a future release.
```
Let's find and fix this before a future upgrade breaks it on us.

Start with desispec.test.test_extract and reproduce the reported pkg_resources DeprecationWarning. Search the resource paths used by that test for relative or absolute components, then run the test again to confirm the warning is gone and the resource lookup still succeeds.

Written by the indexing model from the issue text.