Replace config loader with a new loader

[wata727]

This PR replaces the current config loader that depends on the Terraform internal APIs with a new loader. The new loader decodes the minimum required configuration, so this is making it more robust against schema changes due to new Terraform features. Also, a minimal code base improves maintainability.

An important change is the position of the terraform package. Previously, the terraform package was treated as a copy of the Terraform internal packages, but now it is a unique package that reproduces Terraform semantics.

By extending this package, we can add unique features for TFLint while preserving Terraform semantics. for example,

• Extended module loader enables inspection of local modules without running terraform init.
• Extended config loader allow inspections that are not limited to a single directory (e.g. recursive inspection).
• Extended evaluator gives you fine control over the evaluation of context-sensitive expressions such as path.*.

This PR is the first step towards realizing these features.

In this change, we will try to keep the Loader interface as much as possible. Ideally, the new terraform package could be used to bring together concerns about other Terraform semantics, but that's a future improvement.