The builtin runtime package definitions intentionally diverge from the
actual runtime package's, but this only works as long as they never
overlap.

To make it easier to expand the builtin runtime package, this CL now
loads their definitions into a logically separate "go.runtime"
package.  By resetting the package's Prefix field to "runtime", any
references to builtin definitions will still resolve against the real
package runtime.

Fixes #14482.

Passes toolstash -cmp.

Change-Id: I539c0994deaed4506a331f38c5b4d6bc8c95433f
Reviewed-on: https://go-review.googlesource.com/37538
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>