Artifact debug repr should include the owner

As of bf4123df23b5f93e572cd920f15afba340f92391, `Artifact` equality takes the artifact owner into account. Since the Starlark `repr` of `Artifact` only prints the execpath, it's possible to have two artifacts, `a` and `b`, that have the same execpath (and thus repr) but satisfy `a != b`. Unsurprisingly, this generates confusion–[even for seasoned Blaze developers](https://github.com/bazelbuild/bazel/pull/5814#issuecomment-422268403). Therefore, I propose including the owner in the output of `print`ing an artifact.

Start by locating the Starlark Artifact repr implementation and the owner-aware equality change referenced in the issue. Confirm the work is done when artifacts with the same execpath but different owners produce distinguishable printed representations.

Written by the indexing model from the issue text.