anv: Import mako templates only during execution of anv_extensions

anv_extensions usage from anv_icd was bringing the unwanted dependency
of mako templates for the latter. We don't want that since it will
force the dependency even for distributable tarballs which was not
needed until now.

Jason suggested this approach.

v2: Patch simplification (Jason).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104551


Fixes: 0ab04ba9 ("anv: Use python to generate ICD json files")
Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit a1901d09)