url: new package

url: new package
This is just moving the URL code from package http into its own package,
which has been planned for a while.
Besides clarity, this also breaks a nascent dependency cycle the new template
package was about to introduce.

Add a gofix module, url, and use it to generate changes outside http and url.

Sadness about the churn, gladness about some of the naming improvements.

R=dsymonds, bradfitz, rsc, gustavo, r
CC=golang-dev
https://golang.org/cl/4893043