cmd/compile: only avoid escaping package paths for "go.shape"

We have code that intends to avoid escaping the package path for
built-in packages. But it is hard to determine which packages are
built-in from a general rule, and we really only want to avoid escaping
for the "go.shape" package (since that gives ugly shape type names). So,
fix the code to only avoid escaping the package path specifically for
the "go.shape" package.

Fixes #50200

Change-Id: Ibaedd7690b99a173007c608c5dfa783ef82b326d
Reviewed-on: https://go-review.googlesource.com/c/go/+/372934


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>