addrs: Be explicit about checkable object address kinds

Previously we were attempting to infer the checkable object address kind
of a given address by whether it included "output" in the position where
a resource type name would otherwise go.

That was already potentially risky because we've historically not
prevented a resource type named "output", and it's also a
forward-compatibility hazard in case we introduce additional object kinds
with entirely-new addressing schemes in future.

Given that, we'll instead always be explicit about what kind of address
we're storing in a wire or file format, so that we can make sure to always
use the intended parser when reading an address back into memory, or
return an error if we encounter a kind we're not familiar with.

internal/addrs/checkablekind_string.go

@@ -0,0 +1,33 @@
+// Code generated by "stringer -type=CheckableKind check.go"; DO NOT EDIT.
+
+package addrs
+
+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[CheckableKindInvalid-0]
+	_ = x[CheckableResource-82]
+	_ = x[CheckableOutputValue-79]
+}
+
+const (
+	_CheckableKind_name_0 = "CheckableKindInvalid"
+	_CheckableKind_name_1 = "CheckableOutputValue"
+	_CheckableKind_name_2 = "CheckableResource"
+)
+
+func (i CheckableKind) String() string {
+	switch {
+	case i == 0:
+		return _CheckableKind_name_0
+	case i == 79:
+		return _CheckableKind_name_1
+	case i == 82:
+		return _CheckableKind_name_2
+	default:
+		return "CheckableKind(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+}