main/dovecot: fix recursive dependency with dovecot-pigeonhole-plugin
dovecot-pigeonhole-plugin provides so:libdovecot-sieve.so, managesieve needs so:libdovecot-sieve.so. If managesieve is not shipped in the dovecot-pigeonhole-plugin subpackage then dovecot depends on it but since dovecot-pigeonhole-plugin already depends on dovecot we have a recursive dependency.
See #13173 (closed)