Added a check for unused imports

src/build/errors/UnknownPackage.go

@@ -0,0 +1,18 @@
+package errors
+
+import "fmt"
+
+// UnknownPackage represents unknown package errors.
+type UnknownPackage struct {
+	Name        string
+	CorrectName string
+}
+
+// Error generates the string representation.
+func (err *UnknownPackage) Error() string {
+	if err.CorrectName != "" {
+		return fmt.Sprintf("Unknown package '%s', did you mean '%s'?", err.Name, err.CorrectName)
+	}
+
+	return fmt.Sprintf("Unknown package '%s'", err.Name)
+}

src/build/errors/UnusedImport.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// UnusedImport error is created when an import is never used.
+type UnusedImport struct {
+	Package string
+}
+
+// Error generates the string representation.
+func (err *UnusedImport) Error() string {
+	return fmt.Sprintf("Unused import '%s'", err.Package)
+}