fix(spdxexp): keep LicenseRef leaves when expanding OR

[MrBeldum]

Summary

expandOrTerm only kept isLicense() leaves when flattening an OR. A LicenseRef-* alternative was dropped before Satisfies ran, so:

• Satisfies("MIT OR LicenseRef-x", []string{"LicenseRef-x"}) returned false
• Satisfies("MIT AND (LicenseRef-a OR LicenseRef-b)", []string{"MIT"}) returned true (the required refs vanished)

expandAndTerm already kept LicenseRef nodes; this mirrors that in expandOrTerm.

Fixes #165.

Test plan

• go test ./... -count=1
• New TestSatisfiesLicenseRefOR and TestExpandOrKeepsLicenseRef cover the failing shapes from Satisfies ignores a LicenseRef alternative of an OR #165

[elrayle]

@MrBeldum Thanks for the proposed fix. To be consistent with existing tests, I added tests to satisfies_test.go under PR #167. Since your proposed fix is just what was needed, I added you as a co-author to that PR.

Closing this PR as superseded by #167.