Use 'GAV' for result keys in LicensedArtifactResolver

[attilapuskas]

Fixes #413

In the result mapping we need to use "GAV" to be compatible with the other classes in the plugin. While the exclude/includeArtifacts should still use the Artifact#getId, since the getDependencyTrail also uses that format, which is considered when we exclude all the transitive deps of excluded artifacts.

Note that the added it is based on the already existing download-licenses-include-exclude-types, but with a small change in the settings to use excludeTransitiveDependencies instead.