`identify.license_id(..)` only supports reporting single licenses in files; files with multiple licenses are not handled properly

[ngie-eign]

Description

Some sources are released with multiple licenses. Core pieces of the Infiniband stack in Linux (for instance) are released under the GPL 2.0 and OpenIB BSD (2-clause) licenses: (example).

Being able to accurately detect that the sources are released under multiple licenses would help the accuracy of the tool.

Further discussion

Some licenses are released under an intersection (AND in SPDX terms) or union (OR in SPDX terms) of multiple licenses: in the above example, either license can be accepted. Distinguishing each case from one another is key to ensure that licensing is being honored appropriately.