Remove Android plugin resolver.

Was only ever there for poor man's testing initially.
Minor refactor to liberate how the plugin repository is created from PluginResolverFactory.
When a plugin was not found, include information in the error message on how to determine what plugins are available.

Right now this is just a link to the userguide for distribution plugins and a link to the Bintray org for repo plugins.
Improve the error message when trying to apply a core plugin with a version number.
Require the all buildscript {} blocks are before plugins {} blocks.
Checkstyle fix.
Provide a specific error message if a user tries to apply a non core plugin that is already on the script classpath.

Only covers the simple, single script, case at the moment.
Test fix after making PluginResolverFactory depend on DocumentationRegistry.
Detect attempts to apply plugins from parent classpath in plugins {} block.