Feature request: include a license summary

[sellout]

Either in the license-report.md or separately, include a list of the distinct licenses in the dependency list.

It might look something like this:

• BSD-2-Clause
• BSD-3-Clause
• ISC
• MIT
• Some package information is MISSING, so this list may be incomplete.

#90 could make this trivial enough that it doesn’t need to be explicitly handled by cabal-plan.