Offer suggestions when license full name is used instead of short name

[hadess]

When building Ghidra for Flatpak, I got:

• tag-invalid : <project_license> is not valid [Apache License 2.0]SPDX ID 'Apache License 2.0' unknown

Ideally, appstream-glib would have known about the full name, and offered up the short name as a suggestion:
https://spdx.org/licenses/Apache-2.0.html

So it would read:

• tag-invalid : <project_license> is not a known short name [Apache License 2.0] use SPDX ID 'Apache-2.0' instead

[hughsie]

We don't know the long name, we only store the short version in the library. The long form is also less "API stable" than the SPDX tag, and so I don't think it's super possible to map back. Sorry.

[hadess]

Would be useful if it at least said that it needs to be a "license expression" which has a very defined format.

The tag name makes it look like free-form text, even if the spec mentions the "license expression" format.