let extract_source = builddir when more than one folders being extracted

[onesuper]

Hi,

Sometimes, a tar ball does not retain the parent directory. e.g. After type tar -xvf abc.tar.gz , there are some sub directories under the builddir like src, doc... instead of the abc folder.

The original code chooses the latest created folder after extraction to be the working directory for the building stage. I guess it makes more sense if we could return builddir to handle the above case (when more than one folders are extracted).

The new code return Dir.pwd directly, since already the working dir has been changed to builddir. And I make no changes to the tests since the change itself didn't break the test.

Regards
Yichao

[onesuper]

@bernd Not sure whether the change will affect other use case.