Improve package and directory structure

[L-Lavigne]

Our package structure does not match our directory structure, for example in the file modules/server/src/main/scala/GrpcConfigServer.scala:

package mu.rpc
package server

This causes various inconveniences such as ScalaTest/Specs2 integration in IntelliJ IDEA being unable to identify and run tests together from the src/test roots.

We also use chained package declarations which is somewhat non-standard, and it might be worth revisiting that choice.

A re-arrangement of packages would also be a good time to change the package roots once we've agreed internally on a naming convention for higherkindness.

The same points currently apply to https://github.com/higherkindness/skeuomorph.