addrs: Target is a fmt.Stringer

We have some test code that includes *addrs.Target values in fmt calls with
the %s and %q verbs, but that type was not actually a fmt.Stringer before.

Go 1.26 is introducing some new checks that cause those uses to cause
failures when building those tests. We could potentially change the tests
to produce the string representation in a different way, but we typically
expect our address types to be "stringable" in this way, so instead we'll
just make Target be a fmt.Stringer, delegating to the String method of
the underlying addrs.Targetable implementation. The addrs.Targetable
interface requires a String method, so all implementations are guaranteed
to support this.

Note that we're not actually using Go 1.26 yet at the time of this commit,
but this is an early fix to make the upgrade easier later. I verified this
using go1.26rc1.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/addrs/parse_target.go

@@ -445,3 +445,10 @@ func (t *Target) ModuleAddr() ModuleInstance {
 		panic(fmt.Sprintf("unsupported target address type %T", addr))
 	}
 }
+
+func (t *Target) String() string {
+	if t == nil || t.Subject == nil {
+		return "<nil>"
+	}
+	return t.Subject.String()
+}