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>

src/cmd/compile/internal/types/pkg.go

@@ -9,6 +9,7 @@ import (
 	"cmd/internal/objabi"
 	"fmt"
 	"sort"
+	"strings"
 	"sync"
 )
 
@@ -48,7 +49,13 @@ func NewPkg(path, name string) *Pkg {
 	p := new(Pkg)
 	p.Path = path
 	p.Name = name
-	p.Prefix = objabi.PathToPrefix(path)
+	if strings.HasPrefix(path, "go.") {
+		// Special compiler-internal packages don't need to be escaped.
+		// This particularly helps with the go.shape package.
+		p.Prefix = path
+	} else {
+		p.Prefix = objabi.PathToPrefix(path)
+	}
 	p.Syms = make(map[string]*Sym)
 	pkgMap[path] = p
 

src/cmd/compile/internal/types/type.go

@@ -2202,4 +2202,4 @@ var (
 
 var SimType [NTYPE]Kind
 
-var ShapePkg = NewPkg(".shape", ".shape")
+var ShapePkg = NewPkg("go.shape", "go.shape")