cmd/go: split out cmd/go/internal/web

cmd/go: split out cmd/go/internal/web

This is one CL in a long sequence of changes to break up the
go command from one package into a plausible group of packages.

This sequence is concerned only with moving code, not changing
or cleaning up code. There will still be more cleanup after this sequence.

The entire sequence will be submitted together: it is not a goal
for the tree to build at every step.

For #18653.

Change-Id: I2f349150659b6ddf6be4c675abba38dfe57ff652
Reviewed-on: https://go-review.googlesource.com/36201
Reviewed-by: David Crawshaw <crawshaw@golang.org>