cmd/compile: fix up shape type package

Use go.shape instead of .shape as the package the compiler uses
to store shape types.

Prevent path escaping for compiler-internal types, so we don't
need to see %2e everywhere.

Change-Id: I98e39c3b6472560113bdea7e0ba6eb7b81cb35e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/361174


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>