Fix merge conflict
Fix the following:
- In the SPDXParser, when setting files, the old files are not completely removed (changes to the file SPDXDocument.java)
- The SpdxLicenseMapper only contains mappings for non standard licenses which have been added to the outputDocument.  I think that the map should also contain mappings for any non standard licenses which already exist.  I added a new method to the mapper to add existing licenses and updated the SpdxLicenseInfoMerger to call that method if a match was found.

Signed-off-by: goneall <gary@sourceauditor.com>