cmd/compile: permit Unicode spaces in (expanded) package paths

This doesn't change the existing restriction with disallows
spaces in import paths (as found in an import declaration).
It simply permits packages to be under a directory name that
may contain spaces.

Verified manually that it works. This could use a test, but the
change is trivial. We also can't use the existing test framework
(under test/) because the way those tests are run with test/run.go,
the mechanims for compiling a directory, even if it contains blanks
it its name, does't produce compiler paths with blanks
(the compilation is local).

Fixes #20306.

Change-Id: I6cbffb86c3394347897c3c94b110da0aadc5bfdf
Reviewed-on: https://go-review.googlesource.com/46001Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/compile/internal/gc/bimport.go

@@ -285,7 +285,7 @@ func (p *importer) pkg() *types.Pkg {
 	}
 
 	// we should never see a bad import path
-	if isbadimport(path) {
+	if isbadimport(path, true) {
 		p.formatErrorf("bad package path %q for package %s", path, name)
 	}
 

src/cmd/compile/internal/gc/main.go

@@ -891,7 +891,7 @@ func importfile(f *Val) *types.Pkg {
 		return nil
 	}
 
-	if isbadimport(path_) {
+	if isbadimport(path_, false) {
 		return nil
 	}
 
@@ -935,7 +935,7 @@ func importfile(f *Val) *types.Pkg {
 		}
 		path_ = path.Join(prefix, path_)
 
-		if isbadimport(path_) {
+		if isbadimport(path_, true) {
 			return nil
 		}
 	}

src/cmd/compile/internal/gc/subr.go

@@ -1969,7 +1969,7 @@ var reservedimports = []string{
 	"type",
 }
 
-func isbadimport(path string) bool {
+func isbadimport(path string, allowSpace bool) bool {
 	if strings.Contains(path, "\x00") {
 		yyerror("import path contains NUL")
 		return true
@@ -1998,7 +1998,7 @@ func isbadimport(path string) bool {
 			return true
 		}
 
-		if unicode.IsSpace(r) {
+		if !allowSpace && unicode.IsSpace(r) {
 			yyerror("import path contains space character: %q", path)
 			return true
 		}