Consider package context and default licenses to resolve unknown licenses and license versions (like for the GPL)

[pombredanne]

We should design a way to reuse a package context (with its declared, top level licensing) to inform the resolution of mildly ambiguous license and unknown references found in the same code tree.
This would put in code the principle that a top level; declared licenses applies to the package unless otherwise noted and that it also informs some ambiguities.

For instance, say we have a package with a top level GPL-2.0 license. The file-level notices of license under the GPL could be resolved to GPL-2.0 instead of GPL-1.0-or-later. This is akin to resolve a file reference, but using a top level license instead, and a slightly broader approach.

We could also provide a CLI option to provide the default as an expression.