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/proto/aaa/aaa.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+option go_package = "github.com/moul/protoc-gen-gotemplate/examples/single-package-mode/output/aaa";
+
+package the.aaa.package;
+
+message AaaRequest {
+  string blah = 1;
+}
+message AaaReply {
+  string error = 1;
+}