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.
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