Forbid underscore as rule name

[anderseknert]

Harmless, but still, a wildcard rule name doesn't make sense 😄

package p

_ if true

opa eval -f pretty -d p.rego data.p
{
  "$0": true
}

[anderseknert]

Had a look at this, and have some code I can push to resole the issue. Had a test failure here though, and that test clearly suggests intention to allow _ as a rule name. I'm not sure why this would be though 🤔

opa/v1/ast/parser_test.go

Lines 2267 to 2274 in 2136920

	assertParseRule(t, "wildcard name", `_ { x == 1 }`, &Rule{
	Head: &Head{
	Name: "$0",
	Reference: Ref{VarTerm("$0")},
	Value: BooleanTerm(true),
	},
	Body: MustParseBody(`x == 1`),
	})