`.short_path` is not usable for output artifacts in external repository

If I create an artifact with

```
ctx.actions.declare_file("out")
```

In package `@l//lp`, its various paths will be as follows:

`.path`: `bazel-out/k8-fastbuild/bin/external/l/lp/out`
`.root.path`: `bazel-out/k8-fastbuild/bin`
`.short_path`: `../l/lp/out`

`.short_path` is not really usable: it's probably relative to the `bin` directory of rules in the main repository, but since that directory is not available anywhere (`ctx.bin_dir` yields `bazel-out/k8-fastbuild/bin`), there is not a lot I can do with it.

It's also not "relative to its root", which is what the documentation says.

(somewhat related to #3675)

/cc @haberman

Start with the ctx.actions.declare_file artifact and the ctx.bin_dir entry point, comparing the reported .path, .root.path, and .short_path values for an external repository. Trace the documented “relative to its root” behavior and add coverage showing the expected usable path for this case.

Written by the indexing model from the issue text.