[dev.typealias] cmd/compile: type-check type alias declarations

Known issues:
- needs many more tests
- duplicate method declarations via type alias names are not detected
- type alias cycle error messages need to be improved
- need to review setup of byte/rune type aliases

For #18130.

Change-Id: Icc2fefad6214e5e56539a9dcb3fe537bf58029f8
Reviewed-on: https://go-review.googlesource.com/35121
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -927,7 +927,7 @@ func mkpackage(pkgname string) {
 				continue
 			}
 
-			if s.Def.Sym != s && s.Flags&SymAlias == 0 {
+			if s.isAlias() {
 				// throw away top-level name left over
 				// from previous import . "x"
 				if s.Def.Name != nil && s.Def.Name.Pack != nil && !s.Def.Name.Pack.Used && nsyntaxerrors == 0 {