loader: load packages using Go modules

This commit replaces the existing ad-hoc package loader with a package
loader that uses the x/tools/go/packages package to find all
to-be-loaded packages.

loader/errors.go

@@ -1,10 +1,5 @@
 package loader
 
-import (
-	"go/token"
-	"strings"
-)
-
 // Errors contains a list of parser errors or a list of typechecker errors for
 // the given package.
 type Errors struct {
@@ -15,25 +10,3 @@ type Errors struct {
 func (e Errors) Error() string {
 	return "could not compile: " + e.Errs[0].Error()
 }
-
-// ImportCycleErrors is returned when encountering an import cycle. The list of
-// packages is a list from the root package to the leaf package that imports one
-// of the packages in the list.
-type ImportCycleError struct {
-	Packages        []string
-	ImportPositions []token.Position
-}
-
-func (e *ImportCycleError) Error() string {
-	var msg strings.Builder
-	msg.WriteString("import cycle:\n\t")
-	msg.WriteString(strings.Join(e.Packages, "\n\t"))
-	msg.WriteString("\n at ")
-	for i, pos := range e.ImportPositions {
-		if i > 0 {
-			msg.WriteString(", ")
-		}
-		msg.WriteString(pos.String())
-	}
-	return msg.String()
-}