sourceLibrary should not compile in unused files

[Geod24]

System information

• dub version: DUB version 1.17.0, built on Sep 7 2019
• OS Platform and distribution: Any
• compiler version: Any

Bug Description

Currently, sourceLibrary referenced by a project will be fully included in this project.
This means that, for example, if a library provides binding to a C library it uses internally,
but those bindings are never used by the application, it will still need to link in this library.
This result in longer compilation time, larger executable, and increased number of dependencies.

How to reproduce?

Create a sourceLibrary, make it depend on a C library, then compile your project that depends on it without including anything from it.

Expected Behavior

rdmd-style approach to dependency.