(gnus-namazu/group-prefixed-name): Removed.

(gnus-namazu/group-prefixed-name): Removed.
(gnus-namazu/real-group-name): Removed.
(gnus-namazu/decode-group-name): Removed.
(gnus-namazu/directory-table): New internal variable.
(gnus-namazu/make-directory-table): New function.
(gnus-namazu/search): Call the above function, and use
`gnus-namazu/directory-table' to get a real group name from a file
name.
(gnus-namazu-create-index): Call `gnus-namazu/make-directory-table'.
(gnus-namazu/update-sentinel): Ditto.
(gnus-namazu-update-all-indices): Simplified.
(gnus-namazu-update-indices): New function.