Improved license detection for the guess-license target.

Before, the first file that looked like a license file was considered.
The others were completely ignored. This led to a wrong license for
cross/arm-none-eabi-gcc. To prevent these cases in the future, the license
is only guessed if there is exactly one file with a typical license name.

This approach is still naive, but at least a little more precise. Replacing
the guess-license with a determine-licenses is much more complicated
though, since each source code file may have its own license declared, and
handling all these special cases leads to very complex license expressions
(like "gnu-gpl-v3 for all files, except for special.c, which is apache-2.0
or mit). This is very hard to do correctly.

(rillig)