Move useful code out of CLI tool and into shared library

[ebkalderon]

I think it might be useful to move as much code as possible out from the Amethyst build tool into an amethyst_tools library target. This has several advantages:

• Build tool becomes thin frontend for library
• Project management and Cargo wrapper code is reused when building future tools
• Integration tests can be written directly in Rust without having to rely on shell scripts

The new source folder structure would look like this:

• src/
  • bin/
    • amethyst.rs
    • <tool>.rs
  • lib.rs
  • <stuff>.rs

This issue is blocked by pull request #33, which will make this much easier to accomplish.

EDIT: Looks like this change will actually be coming together with pull request #33!

[ebkalderon]

@amethyst/tools-devs I am no longer sure if this will be needed. If we implement proper project detection and validation in the engine, would there be a need to keep this a separate library? Why not import this functionality from the engine?

[ebkalderon]

@amethyst/tools-devs I am no longer sure if this will be needed. If we implement proper project detection and validation in the engine, would there be a need to keep this a separate library? Why not import this functionality from the engine?