Populate "package notice text" using NOTICE* files text

[pombredanne]

We should do a better job to populate "package notice text" field of a Package using NOTICE* files text, and related key files notices that we detect.

See also:

• Collect and attach NOTICEs content to the packages scancode-toolkit#3511

[DennisClark]

please concatenate the texts if multiple "key" files are detected, with the NOTICE file first if there is one.

[tdruez]

Merged and deployed.