Build traceur with --referrer names.

Prefix traceur names with traceur@0.0.Y/ then
simulate 'paths' by removing the same prefix in locate().
The first build uses traceur@0.0.X/, the second one
uses traceur@0.0.Y/, so the input and output names are
disjoint.

Prefix calls in test/unit/* to System.get() with 'traceur@0.0.Y/

Review URL: https://github.com/google/traceur-compiler/pull/651

Closes #651.