doxygen: Correct TAGFILE linkage of main

doxygen: Correct TAGFILE linkage of main

core.doxy was renamed to main.doxy, along with output folder in
the below 2004 commit.

Correct the other modules' TAGFILE linkage to find the main folder.

  commit 3ef972f538e1ff207f5919a8068bf14af18fb270
  Author: Brian Paul <brian.paul@tungstengraphics.com>
  Date:   Sun May 16 22:07:02 2004 +0000

      Replaced 'core' with 'main'.
      Other minor updates.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>