publican: support out-of-source tree build

When srcdir!=builddir, there is no way to tell publican that the source
is in srcdir rather than builldir. The workaround is to copy the source
files from srcdir to builddir. To retain the en-US final destination
name, the source directory is renamed to en_US.

Tested-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>