None of the packages in ROS 2 have a dependency on the colcon build tool. This is sort of nice since production systems don’t necessarily need build tools so ROS 2 is one step closer to minimal production environments if the functionality to split out development info from runtime libraries is contributed.
The Linux development instructions cover the colcon package installation with python3-colcon-common-extensions. The package name isn’t my favorite. (My preferred color of the bikeshed is colcon-sensible 
) but because colcon is primarily implemented via plugins to the core colcon package and usable beyond the ROS ecoystem, the colcon-ros package includes only the packages needed to build ROS packages and not any of the creature comfort plugins that you might also want. Hence colcon-common-extensions to wrap them both.
I don’t agree that colcon or any tools needed only at build time ought to be in ros-base. But I also see where the suggestion is coming from and I think a base-devel- or build-essential-like metapackage which could include everything necessary to build ros-base from source is a reasonable metapackage to propose. As well as the inclusion of said package in ros-desktop. The metapackage contents are specified in REP-2001 and you can open pull pull requests against the ros-infrastructure/rep project to propose modifications.