New plans.Quality type for display-relevant facts about a plan

This commit replaces the existing jsonformat.PlanRendererOpt type with a new
type with identical semantics, located in the plans package.

We needed to be able to exchange the facts represented by
`jsonformat.PlanRendererOpt` across some package boundaries, but the jsonformat
package is implicated in too many dependency chains to be safe for that purpose!
So, we had to make a new one. The plans package seems safe to import from all
the places that must emit or accept this info, and already contains plans.Mode,
which is effectively a sibling of this type.

internal/plans/quality_string.go

@@ -0,0 +1,24 @@
+// Code generated by "stringer -type Quality"; DO NOT EDIT.
+
+package plans
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[Errored-0]
+	_ = x[NoChanges-1]
+}
+
+const _Quality_name = "ErroredNoChanges"
+
+var _Quality_index = [...]uint8{0, 7, 16}
+
+func (i Quality) String() string {
+	if i < 0 || i >= Quality(len(_Quality_index)-1) {
+		return "Quality(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _Quality_name[_Quality_index[i]:_Quality_index[i+1]]
+}