runfiles symlink obscured

### Description of the problem / feature request:

There seems to be a hidden requirement for `data` files, namely that the target label must not be a prefix of the runfile. If it is, we get a warning like

```
WARNING: /Users/gar/mina/ppx_optcomp/test/BUILD.bazel:21:9: runfiles symlink test/import_relativity/exit.mlh -> test/import_relativity/exit.mlh obscured by test/import_relativity -> bazel-out/darwin-fastbuild/bin/test/import_relativity
```

and failure, at least for test rules.

If I were not familiar with test rule implementation details I think this would be indecipherable. I suggest:

* document this in the [`data` attribute documentation ](https://docs.bazel.build/versions/master/be/common-definitions.html#common-attributes), not just the runfiles docs.
* add a suggestion to the warning message, something like "Make sure your target name is not a prefix of a data file."
* even better, eliminate the problem - do not symlink the target label name, generate something unique.

### Feature requests: what underlying problem are you trying to solve with this feature?

Avoid surprises.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I have a test rule in package //test with `name="import_relativity"`, and `data=["import_relativity/entry.mlh"]`. In the rule implementation, I iterate over `ctx.files.data`, adding each to `ctx.runfiles`. What gets added is `test/import_relativity/entry.mlh`. The test fails when run with file not found for the `entry.mlh` file, plus the warning noted above. This is apparently because a symlink is created from the target label, `test/import_relativity`, which "obscures" the runfile symlink. If I change the name of the rule, e.g. to `import_relativity_test`, then the test succeeds.

### What operating system are you running Bazel on?

MacOS Catalina

### What's the output of `bazel info release`?

release 3.6.0

### Have you found anything relevant by searching the web?

No.

Reproduce the failure from the test rule in test/BUILD.bazel at the reported data attribute, using data=["import_relativity/entry.mlh"] and the rule's ctx.files.data-to-ctx.runfiles flow. Read the common data attribute documentation and runfiles documentation, then trace the warning and obscured symlink behavior. Done means the target/data naming case no longer fails, or the documented constraint and warning provide an actionable explanation.

Written by the indexing model from the issue text.