Improve developer experience

[wata727]

The OPA ruleset is developed as a plugin for TFLint, so some features have developer experience issues. For example:

• Creating the policy directory must be done manually with mkdir ./.tflint.d/policies, and typos in the name will be silently ignored.
• There is no easy way to check whether the policy you created is actually activated.
• Tests are enabled by environment variables (TFLINT_OPA_TEST=1) and do not provide detailed reporting on failure.

We need to fix these issues and provide an experience comparable to opa(1) and conftest.

One idea would be to provide a new CLI like tflint-opa and guide users to setup using that.

[KingBain]

just adding to this, its a bit of a PIA how tflint handles config paths.
as mentioned here terraform-linters/tflint#1706 (comment)

but I'm finding its the same for the policies folder. I ended to having to set the environment variable for the opa policies folder at the root of the project

export TFLINT_OPA_POLICY_DIR=$(pwd)/.tflint.d/policies
export TFLINT_CONFIG_FILE=$(pwd)/.tflint.hcl 

then
tflint --format=compact --recursive

can be run and function as expected