Fix multi-module classpath issues (closes #183)

[keeganwitt]

Problems

• Plugin ClassLoader is re-used between modules, so classpath was a super-set of all modules that proceeded it (since dependencies were being added to plugin ClassRealm, which adds to plugin ClassLoader).
• Execute, Console, and Shell mojos didn't pass the ClassLoader to use to GroovyShell, so the plugin ClassLoader was accidentally being used, including for PROJECT_ONLY configuration.

Changes

• Pass ClassLoader to GroovyShell in Execute, Console, and Shell mojos.
• Load project dependencies into a new ClassLoader, instantiated per mojo execution instead of using Plexus Classworld.