scala_library with generated resources and no srcs fails to build

A `scala_library` rule that has no `srcs` but generated `resources` will fail to build.

Pure resource jars seem to be a valid use-case, and indeed rules_scala seems to support them if the resource is not generated but a plain file.

Consider the following example
```
scala_library(
name = "lib",
resources = [":generated.txt"],
)
genrule(
name = "generated",
outs = ["generated.txt"],
cmd = "echo test > $(OUTS)",
)
```

Trying to build the `lib` target will fail with the following error.
```
File generated.txt does not seem to exist.
```
Even though building `generated.txt` succeeds and produces an output.

Reproduce the failure by building the `lib` target from the supplied `scala_library` and `genrule` example, then trace how generated `resources` are checked. Done means a `scala_library` with no `srcs` builds successfully when its resource is generated, while plain-file resources remain supported.

Written by the indexing model from the issue text.