Add Regal for linting Rego

[anderseknert]

I think you're all familiar with Regal by now, so I'll skip the introduction :) This PR introduces the OPA community's favorite linter to the Konstraint project, and along with that a bunch of fixes as suggested by the linter.

Regal checks addressed:

• import-after-rule
• line-length
• unconditional-assignment
• use-assignment-operator
• use-in-operator
• use-some-for-output-vars

OPA strict mode compliance:

• Variables must not shadow input

Both have been added to the CI pipeline to ensure future compliance. Make sure to check in every once in a while, as new Regal releases are published quite frequently, and there's often some useful new rule included in each release.

Fixes #446

[jalseth]

LGTM, thanks Anders!

The failing CI is unrelated to this change.

[jalseth]

@anderseknert Scratch that, please rebase on main. I think you will need to generate the Gatekeeper YAMLs as you changed the source and the acceptance tests verify the YAMLs are generated correctly.

[anderseknert]

@jalseth it's bedtime here, but will do tomorrow 👍

[anderseknert]

@jalseth done 👍 make update-static fails with:

$ make update-static
go build -o build/konstraint
./build/konstraint create examples
WARN[0000] Skipping constraint generation due to use of parameters  name=RequiredLabels src=examples/required-labels/src.rego
INFO[0000] completed successfully                        num_policies=20
./build/konstraint create test/create --output test/create
Error: get violations: parse directory: filter rego files: 1 error occurred during loading: stat test/create: no such file or directory

But I was able to resolve that by simply creating the directory first.

There's also a test/doc directory created by running that command, but that does not seem to have been checked in previously, so I did not include that.

[jalseth]

Thanks again!