Clarify metapackage dependency behavior.

Currently, if a user’s package depends on a metapackage (such as fanuc), catkin build displays an error similar to the following:

Abandoned <<< fanuc_motion             [ Depends on unknown jobs: fanuc ]

This is in accordance with REP-140, which says the following with respect to metapackages:

They cannot be used for catkin builds and compile nothing themselves.

Except apparently catkin_make_isolated builds metapackages and packages which are linked only by <exec_depend> dependencies.

So we should either allow or disallow depending on metapackages for building. If we allow it, we need to patch the build verb, and if we continue to disallow it, we need to make this error message clearer so people know it’s because they’re depending on metapackages.

Related discussions: #364 #224 #148

[jbohren]

Is it a good idea to allow this just because catkin_make doesn’t error out at the moment?

I think enough stuff is changing that we don’t need catkin_tools to start enforcing this, and we could switch this from a warning to an error with the 1.0.0 release.

[gavanderhoorn]

Is it a good idea to allow this just because catkin_make doesn’t error out at the moment? Both REP-127 - <metapackage/> and REP-140 - <metapackage/> state (non-meta) packages are not allowed to depend on metapackages. There will probably be some flack if catkin_tools starts enforcing this, but it would seem like an opportunity to do so?