testing/mercury: remove -dev subpackage, depend on libs
The -dev subpackage didn't make a lot of sense, this is a compiler so there's really no situation where you'd want to install/use the compiler without headers that the language requires (e.g. std library).
mercury depending on mercury-libs ensures that libs are installed when building, since mercury doesn't appear to support soname versioning (linking against libgc.so resulted in a binary that failed to find symbols even though libgc.20{,06.1} were installed, for example)