update set for "rust-rebuild" to adapt to changes in gentoo

The virtual package is deprecated and we use an eclass to depend on an
any-of group for the relevant rust compilers. This set no longer does
anything without an update.

Start checking BDEPEND for the specific rust compilers. Also keep
checking for the virtual, so that "rust-rebuild" reports on packages
that have not, in fact, been rebuilt yet.

Bug: https://bugs.gentoo.org/906044
Closes: #1399
Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Mike Gilbert <floppym@gentoo.org>

cnf/sets/portage.conf

@@ -110,8 +110,8 @@ class = portage.sets.dbapi.UnavailableBinaries
 [changed-deps]
 class = portage.sets.dbapi.ChangedDepsSet
 
-# Installed packages for which vdb *DEPEND includes virtual/rust
+# Installed packages for which vdb *DEPEND includes rust
 [rust-rebuild]
 class = portage.sets.dbapi.VariableSet
 variable = BDEPEND
-includes = virtual/rust
+includes = dev-lang/rust dev-lang/rust-bin virtual/rust