Rename en_US to sources

The reason this directory exists is because we need to copy it into
$builddir so we can combine it with generated sources (we can't pass
multiple source paths into publican).

So instead of having en_US, renamed to en-US stop the confusion and rename
the sources to "sources". That gets copied to en-US which will then contain
the actual output.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>