Per-library fallback LAL data path for XLALFileResolvePathLong()

- XLALFileResolvePathLong() replaces 'fallbackdir' with 'fallbackpath'
  - May contain either absolute or relative (to liblalsupport.so) paths
- LALSUITE_WITH_FALLBACK_DATA_PATH (replaces LAL_WITH_DATA_PATH)
  defines LAL_FALLBACK_DATA_PATH to fallback LAL data path
  - Defaults to $(pkgdatadir), same as for 'fallbackdir' argument
- New macro XLAL_FILE_RESOLVE_PATH() passes LAL_FALLBACK_DATA_PATH to
  XLALFileResolvePathLong(), so each library gets its own fallback path
  - Not designed to be used outside of LALSuite; libraries implement their
    own functions (e.g. XLALReadEphemerisFile(), XLALSimReadDataFileOpen()
    which call XLAL_FILE_RESOLVE_PATH() internally
- Non-inline XLALFileResolvePath() and XLALPulsarFileResolvePath() wrappers
  kept to avoid breaking ABI
- .gitlab-ci.yml: add finding data files test to lalpulsar_longtests

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