The `etc/virtual` file declares the default package to be built for
virtual dependencies declared as "virtual?foo" in $depends.
Before this change, the run-time dependency was added as is to the final
binary package but no pkg providing this virtual pkg was built.
With this file we declare the *default* pkg to be built.
NOTE: "virtual?foo" is only applicable to *run* time dependencies, i.e
only those declared in $depends.
The strip-and-debug-pkgs/generate-runtime-deps hooks have been moved
to the pre-pkg stage, and are run before prepare-32bit; this way dependencies
are collected correctly in all cases.
