net/url: disallow raw IPv6 addresses in host

net/url: disallow raw IPv6 addresses in host

RFC 3986 requires square brackets around IPv6 addresses.
Parse's acceptance of raw IPv6 addresses is non compliant,
and complicates splitting out a port.

This is a resubmission of CL 710176 after the revert in CL 711800,
this time with a new urlstrictipv6 godebug to control the behavior.

Fixes #31024
Fixes #75223

Change-Id: I4cbe5bb84266b3efe9c98cf4300421ddf1df7291
Reviewed-on: https://go-review.googlesource.com/c/go/+/712840
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>