Warn when multiple packages have the same name

[drewbanin]

Describe the feature

If multiple packages have the same name, then whichever package is loaded last will clobber the models/macros/etc provided by other identically-named packages. This is basically never desirable, so dbt should error when it happens.

Steps to reproduce

1. Create a project with the name my_project and one model
2. Create a package for that project also named my_project with no models
3. A dbt run from the root package will not find any models.

Expected behavior

If multiple packages in a project have the same name, dbt should fail with an error:

dbt found more than one package with the name "{{ package_name }}" included in this project. Package names must be unique in a project. Please rename one of these packages.

[drewbanin]

closed by #2030