RFC 3986 requires square brackets around IPv6 addresses.
Parse's acceptance of raw IPv6 addresses is non compliant,
and complicates splitting out a port.
Fixes #31024
Fixes #75223
Change-Id: I477dc420a7441cb33156627dbd5e46d88c677f1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/710176
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>