Automatically detect licenses in packages that follow REUSE specification

[lsandig]

The REUSE specification allows developers to specify fine-grained copyright and licensing information, but requires the license texts to be put in a LICENSES/ subdirectory. These files are currently not picked up by the automatic license check.
While the specification does not prevent you from copying the license texts additionally into a top-level LICENSE.md file, this workaround seems to me unnecessarily redundant.
Do you see any reason not to to try a

license_results = LicenseCheck.find_licenses(joinpath(pkgdir, "LICENSES"))

if Failuremode 1 occurs here?

[ericphanson]

I think we wanted to make sure the license was somewhere "obvious" and not buried, so we didn't just check all subdirs. But LICENSES seems obvious enough, seems OK to me