Parser does not handle new SPDX IDs ending in `+`

[kemitchell]

SPDX released a new version of their license list including identifiers for the FSF licenses that include + suffixes in the license identifiers. The expression syntax long allowed appending a + to any identifier to indicate that license or any later version. Now + is also a valid and used character in license identifiers themselves. Including after the new IDs, like GPL-1.0++, with no clear meaning.

The current parser was written back before the new IDs, when it was safe to assume + always appeared as an independent lexical element.