Cleanup

[lihaoyi]

• Privatize everything possible in core.{internal,exec,resolve}
• Rename mill.main.client to mill.client
• Remove duplication from mill.{api,util}.Util, break out a private[mill] mill.util.MillModuleUtil
• Split up mill.client.Util to have a separate mill.client.ClientUtil with Util containing user-facing stuff and ClientUtil containing internal stuff (though it can't be private since java doesn't support private[mill]
• Add basic Scaladoc to the various modules in core/ to try and define what each one is meant to do