app-text/asciidoc: 9.0.0_rc1 version bump

app-text/asciidoc: 9.0.0_rc1 version bump

Note that since reference outputs are now included in the source
distribution, it doesn't make sense anymore to update the test data
outputs.

- Fix the test deps by adding a dependency on app-text/dvisvgm.  This
  did not cause test failures, but did cause "command not found" type
  errors.

- List documentation in $DOCS instead of calling dodocs manually.

- Consistently put $PYTHON_DEPS at the top of the dependency lists.

- Constrain the dev-libs/libxml2 dependency to slot 2.

- Instead of controlling optional runtime deps with USE flags, use
  "optfeature" from the eutils eclass.  While at it, add further calls
  to optfeature for hitherto only indirectly documented optional
  dependencies.

Signed-off-by: Marc Joliet <marcec@gmx.de>
Signed-off-by: Joonas Niilola <juippis@gentoo.org>