Related to, but not exactly, #746
Prompted by discussion on gleam-lang/gleam#1450

Presently the Adding metadata section of the Hex.pm docs contain a licenses field with the documentation:

A list of licenses the project is licensed under. This attribute is required. It is recommended to use SPDX License identifier.

This leaves ambiguous a few items:

• If a list of licenses is specified, should they be interpreted as being anded or ored together? If the answer is one of these, how would I convey the other?
• How do I convey an exception, such as "Apache-2.0 WITH LLVM-exception"
• How do I convery "Apache-2.0 or later"?

In my opinion (which you're free to ignore 😅) the answer to this is to supersede the licenses field with a license_expression field containing an SPDX Expression. SPDX Expression unambiguously convey ands, ors, withs, and "or later" (via -or-later or +, depending on GNU vs non-GNU).