Warning during publish: 'Maven publication 'pluginMaven' pom metadata warnings'

Full warning:

```
> Task :pkl-gradle:generatePomFileForPluginMavenPublication
Maven publication 'pluginMaven' pom metadata warnings (silence with 'suppressPomMetadataWarningsFor(variant)'):
- Variant runtimeElements:
- contains dependencies that cannot be represented in a published pom file.
- dependency on project ':pkl-tools' declared with Gradle attributes
```

Start by reproducing :pkl-gradle:generatePomFileForPluginMavenPublication and inspect the publication configuration for pluginMaven, runtimeElements, and the :pkl-tools dependency. Determine how the Gradle attributes are introduced and whether the warning should be resolved or intentionally suppressed; done when the publish task has an agreed warning-free or documented result.

Written by the indexing model from the issue text.