fix: replace facet's constant by const_get

[doudou]

No description provided.

[chhtz]

LGTM. I guess we didn't hit this, because it is not used during import or build-phase. (N.B., we still check out syskit@master, which appears to be stale ...)

Nevertheless, there are still calls to constant left in other files, e.g.

• tools-syskit/lib/syskit/models/task_context.rb

  Line 54 in 9097fc7

  if constant("::#{name}") == self

  and line 56

• tools-syskit/lib/syskit/roby_app/plugin.rb

  Line 9 in 9097fc7

  const_set old_name, constant(new_constant)

• tools-syskit/lib/syskit/gui/testing.rb

  Line 222 in 9097fc7

  model_object = constant(model_name)

• tools-syskit/lib/syskit/scripts/browse.rb

  Line 45 in 9097fc7

  constant(model_names.first)

[doudou]

we still check out syskit@master, which appears to be stale

Very stale. I sent a message beginning of the year that we would merge the transition-to-runkit branch back into master and that people should track the v2.1.0 tag (which is the current master)