Improve imports support (imports lookup)

* Add `--single-package-mode option`
    * Add `getMessageType` helper
    * Add `getProtoFile` helper
    * Add `examples/single-package-mode` example

examples/single-package-mode/output/bbb/service.go

@@ -0,0 +1,19 @@
+// file generated with protoc-gen-gotemplate
+package bbb
+
+import (
+	"fmt"
+
+	"github.com/moul/protoc-gen-gotemplate/examples/single-package-mode/output/aaa"
+	"golang.org/x/net/context"
+)
+
+type Service struct{}
+
+func (service Service) Aaa(ctx context.Context, input *aaa.AaaRequest) (*aaa.AaaReply, error) {
+	return nil, fmt.Errorf("method not implemented")
+}
+
+func (service Service) Bbb(ctx context.Context, input *BbbRequest) (*BbbReply, error) {
+	return nil, fmt.Errorf("method not implemented")
+}