pkl-gradle: cannot use pkldoc generator on package URIs

[bioball]

This is a regression:

In the following:

pkl {
  pkldocGenerators {
    register("pkldoc") {
      sourceModules = listOf(uri("package://pkg.pkl-lang.org/pkl-pantry/pkl.toml@1.0.0"))
    }
  }
}

The implicit <taskName>GetImports task will attempt to analyze the imports of a package, which fails with:

* What went wrong:
Execution failed for task ':pkldocGatherImports'.
> –– Pkl Error ––
  Module URI `package://pkg.pkl-lang.org/pkl-pantry/pkl.toml@1.0.0` has invalid syntax.

The workaround for now is to set transitiveModules explicitly to something, which disables the GetImports task dependency.

pkl {
  pkldocGenerators {
    register("pkldoc") {
      sourceModules = listOf(uri("package://pkg.pkl-lang.org/pkl-pantry/pkl.toml@1.0.0"))
      transitiveModules = files("some_file.txt")
    }
  }
}

To fix this: we only need to analyze the imports of file: URIs. There is little chance that non-file URIs can import file URIs, because Pkl's default security manager disallows this.