core: warn when merged units have conflicting dependencies

core: warn when merged units have conflicting dependencies

A unit should not Conflict with itself. It also does not make
much sense for a unit to be After or Before itself, or to
trigger itself in some way.

If one of those dependency types is encountered, warn, instead
of dropping it silently like other dependency types.

% build/systemd-analyze verify test/loopy3.service
...
Dependency Conflicts dropped when merging unit loopy4.service into loopy3.service
Dependency ConflictedBy dropped when merging unit loopy4.service into loopy3.service