IMAP voicemail has moved from building all of Asterisk with a separate
build option (with imap files linked to the main binaries) to a separate
module which can be switched in config. (Only one voicemail module is
allowed at a time, if you have multiple of these installed you can
select between them with noload in modules.conf).
Quirks doesn't handle a flavour moving to unflavoured+subpackage; use
@ask-upgrade so that users of the imap flavour (and only them) are
warned about this at update time.
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