Fix bug in location used to check for existing catalogs

[marc-white]

Fixes #263 .

The issue was a stray get_catalog_fp() call that hadn't been converted correctly to a reference to build_base_path.

PR also includes a test for when the catalog.yaml is stored separately to the data directories, and where existing 'version directories' are in the catalog directory (these shouldn't be found/considered).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.89%. Comparing base (008b280) to head (16c09cf). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #264 +/- ## ======================================= Coverage 97.89% 97.89% ======================================= Files 11 11 Lines 807 807 ======================================= Hits 790 790 Misses 17 17 ```

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