Add more validation checks on .yml config file

[Elgarni]

This PR adds two validation checks after parsing gqlgen.yml:

• Check if the config file has two package names defined in the same directory
• Check if the same filename has been used twice. For instance using gql/gen.go for both exec and model fields.

Although this may seem intuitive, but it can get tricky in a few cases.
For instance, the following gqlgen.yml config file:

schema:
- schema.graphql
exec:
  filename: generated/exec.go
  package: graphql
model:
  filename: generated/models.go
resolver:
  filename: generated/resolver.go
  type: Resolver

When generating the code, the following error message shows up:

generating resolver failed: resolver build failed: required package was not loaded: github.com/elgarni/project/generated.User
exit status 2

The message could be more descriptive. Moreover, the code was generated for exec.go and models.go, which will not compile anyways since they have different package names.

After this PR, the error message becomes:

invalid config format: filenames generated/exec.go and generated/models.go are in the same directory but have different package definitions
exit status 1

And no code was generated.

The objective of this PR is to stop execution early on, if there are errors in the config file, and not generating any code unless everything is defined correctly in gqlgen.yml

[vektah]

Needs tests, see config_test.go for some examples