ruby-ng.eclass: handle BDEPEND for EAPI 7

ruby-ng.eclass: handle BDEPEND for EAPI 7

Add support for BDEPEND in EAPI 7.

It is not clear how to handle test dependencies since the test
framework will need to run on the CBUILD host but the code that needs
to run is already cross-compiled for the CHOST. Most likely tests
won't work at all in this case. Add test dependencies to BDEPEND in
EAPI 7 since this matches most closely to how the test will be
executed.

Adjust `ruby_add_bdepend` to add to BDEPEND on EAPI 7. This also
allows us to keep the name for this method. Add a new
`ruby_add_depend` to add to DEPEND in EAPI 7 and newer only. This
allows for the rare case where gems need to link to other gem
code (e.g. dev-ruby/nokogumbo).

Add ruby itself to RDEPEND, BDEPEND, and DEPEND since it is expected
to be present in all three cases.

Signed-off-by: Hans de Graaff <graaff@gentoo.org>