Introduce new license-files field.

Hi, this is a followup pull request to "New optional field: extra-license-files". This patch also introduces the ability to have multiple license files, though following feedback, unlike the previous pull request, this one uses the following approach, quoted from the commit message:

> The existing license-file field is an alias to license-files,
> restricted to only allow one license file to be specified (as is the
> common case). Allowing multiple license files is necessary for some
> licenses, such as the LGPL.

I myself prefer the extra-license-files option, but feel free to merge whichever pull request and close the other one.