bug#40413: Handling source-only dependencies in a generic way

[Leo Famulari]

Currently, both the go-build-system and the cargo-build-system treat
dependencies as source code that is only built when building the top
package in the tree. That is, the package the user requested to build or
install.

But, the two build systems handle things quite differently. It would be
great to have a generic mechanism for this sort of source-code-only
dependency.