Support dual licensing in UI and possibly in search

[isoos]

After dart-lang/pana#997 gets implemented.

[isoos]

We'll need to add the identified license files to the output of pana (alongside of screenshots or other assets), because the license file identification we do at upload time may not be consistent with the licenses identified later on at analysis time. The /license tab should show all of the license files identified during analysis.

[sigurdm]

This is in progress!