Report detected license expressions in ScanCode

[pombredanne]

Multiple licenses can be and are detected together when appropriate rules exist. However, even though we report these with the same detected start and end line and we know internally that they are detected together and we know if they are license choices or conjunctive licenses, we only report discrete and distinct licenses, and do not provide all these detection details.

We should return all the information that we detect, especially the interesting license choices, possibly as SPDX-style license expressions.

This is especially relevant for multiple choices that exists in Qt and similar (see #73)

[pombredanne]

The reporting as a license expression should be a rather straightforward addition to the license detection, since we have already all the data when we match a license RULE.
The logic will be the following: Once we have matched a text region (start/end lines) to a detection rule:

• if there is more than one license in the rule, create a license expression:
  • if the rule is a license_choice: yes, create this expression as joining with an OR all the rules licenses
  • else, create this expression as joining with an AND

At a later stage, we can replace the list of licenses in all rules (with a one time migration script) by a plain license expression and then support more complex expressions such as (XXXX with YYY-exception) or ZZZZ

At a high level, the key thing to understand is that we already match to "expression-like" rules
we just do not expose that info in the scan reports.

[pombredanne]

The latest https://github.com/nexB/license-expression/releases/tag/v0.6 now fully supports expressions "with exceptions" and expressions using license names containing "or later". This is now good enough to be used and support the implementation of expression in ScanCode