Add "imported target" facility

[ethouris]

This should function as having some target to be executed using an unrelated build system. It should, however, function inside Silvercat as a defined target.

For example, if this is a foreign library, define a target of type library with required extra linker flags and dependencies.

If such a target is then used as a dependency of a program or another library, it will behave the same as if the library was defined to be built from exact sources inside this Silverball.