fix: Improve retrieval of precomputed pepolar fieldmaps (#3466)

Discovered while testing precomputed derivatives in aslprep, which
imports `fmriprep.utils.bids.collect_fieldmaps`. See
PennLINC/aslprep#518.

For PEPolar fieldmaps, the fieldmap reference is often
`desc-epi_fieldmap.nii.gz`, as opposed to
`desc-magnitude_fieldmap.nii.gz`. IMO, it would be good to change
`desc-{epi,preproc}_fieldmap.nii.gz` to
`desc-preproc_{magnitude,fieldmap}.nii.gz`, but this is what it is for
now.

Author: effigies

fmriprep/data/fmap_spec.json

@@ -21,7 +21,7 @@
       },
       "magnitude": {
         "datatype": "fmap",
-        "desc": "magnitude",
+        "desc": ["magnitude", "epi"],
         "suffix": "fieldmap",
         "extension": [
           ".nii.gz",