Implement `getModels` from plugin-core

[Vincent Moittie]

Description

The version 0.15.0 of plugin-core come with getModels that permit of each plugin to indicate all models from all files in projects.

In case of Terraform models are defined by all tf files in a folder.

In this first version, we don't want to manage subfolder, only first level file and other terraform file type (tfvars, ...).

The subfolder will be manage by this issue: #37