Ref-head rules aren't currently handled by Regal

[anderseknert]

Running Regal against some modern Rego where ref-head rules are used, I'm seeing some false positives. This isn't particularly strange considering we've not really had those in mind before, but that's definitely something we should do, and I'd suggest going through all rules where this would be relevant and add test cases to assert these rules are dealt with correctly. Example:

p.rego

package p

import future.keywords.if

a["b"] if input.c

$ regal lint p.rego
Rule:         	use-assignment-operator
Description:  	Prefer := over = for assignment
Category:     	style
Location:     	p.rego:5:1
Text:         	a["b"] if input.c
Documentation:	https://docs.styra.com/regal/rules/style/use-assignment-operator

1 file linted. 1 violation found.

This is obviously not right, as there's no explicit assignment happening here. I'm sure there are more false positives in other rules, so let's try to fix this.

[anderseknert]

While this issue is valid, the example above wasn't great, and it took me quite some time to understand why: open-policy-agent/opa#6184