cmd/compile: apply -importmap to imports before checking for package unsafe

There are fewer special cases this way: the import map applies
to all import paths, not just the ones not spelled "unsafe".

This is also consistent with what the code in cmd/go and go/build expects.
They make no exception for "unsafe".

For #13703.

Change-Id: I622295261ca35a6c1e83e8508d363bddbddb6c0a
Reviewed-on: https://go-review.googlesource.com/18438Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -694,7 +694,13 @@ func importfile(f *Val, line int) {
 		errorexit()
 	}
 
-	if f.U.(string) == "unsafe" {
+	path_ := f.U.(string)
+
+	if mapped, ok := importMap[path_]; ok {
+		path_ = mapped
+	}
+
+	if path_ == "unsafe" {
 		if safemode != 0 {
 			Yyerror("cannot import package unsafe")
 			errorexit()
@@ -706,12 +712,6 @@ func importfile(f *Val, line int) {
 		return
 	}
 
-	path_ := f.U.(string)
-
-	if mapped, ok := importMap[path_]; ok {
-		path_ = mapped
-	}
-
 	if islocalname(path_) {
 		if path_[0] == '/' {
 			Yyerror("import path cannot be absolute path")