cmd/dist: copy needed packages from standard library during bootstrap

This allows use of newer math/big (and later debug/pe)
without maintaining a vendored copy somewhere in cmd.

Use for math/big, deleting cmd/compile/internal/big.

Change-Id: I2bffa7a9ef115015be29fafdb02acc3e7a665d11
Reviewed-on: https://go-review.googlesource.com/31010
Reviewed-by: Minux Ma <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

src/cmd/compile/fmt_test.go

@@ -506,9 +506,7 @@ func formatReplace(in string, f func(i int, s string) string) string {
 
 // blacklistedPackages is the set of packages which can
 // be ignored.
-var blacklistedPackages = map[string]bool{
-	"cmd/compile/internal/big": true,
-}
+var blacklistedPackages = map[string]bool{}
 
 // blacklistedFunctions is the set of functions which may have
 // format-like arguments but which don't do any formatting and
@@ -537,7 +535,7 @@ func init() {
 // To print out a new table, run: go test -run Formats -v.
 var knownFormats = map[string]string{
 	"*bytes.Buffer %s":                                "",
-	"*cmd/compile/internal/big.Int %#x":               "",
+	"*math/big.Int %#x":                               "",
 	"*cmd/compile/internal/gc.Bits %v":                "",
 	"*cmd/compile/internal/gc.Field %p":               "",
 	"*cmd/compile/internal/gc.Field %v":               "",