External SPDX files appear in the files section

[DaveTryon]

This is a known bug but I was unable to find an issue on it. When building an SPDX file, external doc refs gets reported twice--once (correctly) in as an ExternalDocRef, and once (incorrectly) in the files section. The external SPDX file should only be listed in the files section if it is actually shipped with the product.

[DaveTryon]

Dupe of #956

[DaveTryon]

Reopening since it's a different scenario from #956, although probably a common root cause

[sfoslund]

Discussed this in triage and assigning to @pragnya17 since this seems closely related to another issue assigned to her, #956