Selecting among ros versions does not make much sense anymore, so drop the
corresponding package alternative and use whatever system version is
there (see comments in issue #391). System packages are searched in
/opt/ros/{noetic,melodic} only, in this order, but this can be tuned in
user configuration if needed.

Individual DEPEND_ABI.* can still be used for packages that have specific
requirements. They have be set to match any ros version more recent than
'lunar' by default.