ldap: Fix cyclic dependency

lib-dict: Moved dict-ldap to lib-dict-extra This also allows moving lib-ldap away from LIBDOVECOT_SUBDIRS in src/Makefile.am, which was wrong because it's not really part of libdovecot.la.

Split parts of lib-dict into lib-dict-extra. Otherwise there's a circular dependency because lib-dict/dict-fs.c depends on lib-fs, while lib-fs/fs-dict.c depends on lib-dict. This becomes a problem when compiling --without-shared-libs, although for some reason it works for me while linking the Dovecot core, but not when linking external plugins.