The newest ROS2 releases introduced a new build system called `colcon`.
Add a new plugin to support it. Leave it in beta for now until it gets
some mileage.
ant, maven and gradle plugins: use correct defaults for jre (#2453)
Ensure that the expected java version is properly set when the default
openjdk option is used so that the globing mechanism can format the
path to the java binary correctly.
Also ensure that the system works end to end for core16 and core18,
removing the typo of core19 which was supposed to be core16.
Finally, (re-)add the gradle tests to the testing spread.
pluginhandler: handle removal of inconsistent files (#2450)
When a part is cleaned, the system does its housekeeping by removing
all the files or directories it prepared for stage or prime. If the
user manually removes a file (say, in another part's override-prime),
we should ignore the non-existent file and issue a warning instead
of halting and catching fire.
