recipetool: create: split guess_license function

recipetool: create: split guess_license function

The npm recipetool handler redefines the license code the could be
unified. In order to do this refactoring, extract the bits we'll
need into separate functions.

guess_license() is renamed to find_licenses() and is split into
find_license_files() and match_licenses().

Signed-off-by: Enguerrand de Ribaucourt <enguerrand.de-ribaucourt@savoirfairelinux.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>