Address build warnings

[alexander-yevsyukov]

When publishing the following output is given to the console:

> Task :plugin:publishPlugins
Could not reuse POM from pluginMaven publication because withDependencies() was used
The withDependencies block is deprecated and will be removed in the next version of the plugin-publish plugin. Please use the java-gradle-plugin and maven-publish plugins to customize the POM instead.
Ignoring unknown artifact type with extension "jar" and classifier "pure"
You can only upload normal jars, sources jars, javadoc jars and groovydoc jars
to the plugin portal at this time.
Ignoring unknown artifact type with extension "jar" and classifier "test"
You can only upload normal jars, sources jars, javadoc jars and groovydoc jars
to the plugin portal at this time.
Publishing plugin io.spine.tools.gradle.bootstrap version 1.5.8

This is caused by the following code in plugin.gradle:

project.pluginBundle {
    // ...
    it.withDependencies { it.clear() }
    // ...
}

We perform this action to clear POM dependencies. But this technique is already deprecated.

I couldn't find an alternative, but I didn't spend much time on it.

[alexander-yevsyukov]

@dmdashenkov, please see if it's closed.

[dmdashenkov]

This will be postponed. There's no obvious way to do what the warning suggests. If and when we start publishing to JCenter, eliminating the need for a fat Jar, we'll be able to fix this warning. For now, I suggest we leave this as is.