Keep source of external packages in *build* tree

[evaleev]

cleaner, easier to work with multiple build dirs

[justusc]

What is the issue with current setup?

[evaleev]

aesthetics: source pollution (extra effort needed to avoid indexing, etc.), extra effort needed to work with branches (switching branches in the source may require reclone which only happens by doing a build).

I'd say the ExternalPackage mechanism of cmake calls for keeping stuff in build directory. If we used git modules the story would change.

[justusc]

The original intent was to avoid having to pull these dependencies for multiple builds. However, the tagging system that CMake uses to track changes in these dependencies does not allow that behavior.

Given this and your "pain points" it looks like a good change.