fdo#70393: move poppler to a subdir of external

Change-Id: I399df92d2b6907c129e18f3a55afeadf02783c14
Reviewed-on: https://gerrit.libreoffice.org/6548
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>