Terraform JSON Syntax is not supported

[Terraform also supports an alternative syntax that is JSON-compatible](https://www.terraform.io/docs/language/syntax/json.html). Terraform expects native syntax for files named with a .tf suffix, and JSON syntax for files named with a .tf.json suffix.

However, this syntax is not supported by dependabot. As an example:

```json
{
"module": [
{
"kernel": [
{
"source": "git::https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git?ref=v5.0"
}
]
}
]
}
```

is a simple terraform module in JSON syntax that should get upgraded. I've tried both `.json`, and `.tf.json` extensions: https://github.com/captn3m0/dependabot-github-test/tree/main/json, but it doesn't get picked up. However, the plain `.tf` syntax does: https://github.com/captn3m0/dependabot-github-test/blob/main/kernel/main.tf (PR at https://github.com/captn3m0/dependabot-github-test/pull/7)

Start by reproducing the issue with the linked dependabot-github-test examples, comparing the .tf and .tf.json module files and the existing upgrade PR. Trace how Dependabot discovers and parses Terraform manifests; done means the JSON-syntax module is picked up and its dependency can be upgraded.

Written by the indexing model from the issue text.