We move dovecot libexec binaries and modules (including the package
pigeonhole) to their standard locations.  This is okay for libexec
because it was added to core prior to 17.04 release.  Moving the
binaries normally in libexec eliminates the reason the modules were in an
unusual directory, which is that there were conflicts with
libexec binaries that were in lib, which the modules normally use.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>