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/aaa/aaa.pb.go

@@ -0,0 +1,84 @@
+// Code generated by protoc-gen-go.
+// source: aaa/aaa.proto
+// DO NOT EDIT!
+
+/*
+Package aaa is a generated protocol buffer package.
+
+It is generated from these files:
+	aaa/aaa.proto
+
+It has these top-level messages:
+	AaaRequest
+	AaaReply
+*/
+package aaa
+
+import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
+import math "math"
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
+type AaaRequest struct {
+	Blah string `protobuf:"bytes,1,opt,name=blah" json:"blah,omitempty"`
+}
+
+func (m *AaaRequest) Reset()                    { *m = AaaRequest{} }
+func (m *AaaRequest) String() string            { return proto.CompactTextString(m) }
+func (*AaaRequest) ProtoMessage()               {}
+func (*AaaRequest) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+
+func (m *AaaRequest) GetBlah() string {
+	if m != nil {
+		return m.Blah
+	}
+	return ""
+}
+
+type AaaReply struct {
+	Error string `protobuf:"bytes,1,opt,name=error" json:"error,omitempty"`
+}
+
+func (m *AaaReply) Reset()                    { *m = AaaReply{} }
+func (m *AaaReply) String() string            { return proto.CompactTextString(m) }
+func (*AaaReply) ProtoMessage()               {}
+func (*AaaReply) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{1} }
+
+func (m *AaaReply) GetError() string {
+	if m != nil {
+		return m.Error
+	}
+	return ""
+}
+
+func init() {
+	proto.RegisterType((*AaaRequest)(nil), "the.aaa.package.AaaRequest")
+	proto.RegisterType((*AaaReply)(nil), "the.aaa.package.AaaReply")
+}
+
+func init() { proto.RegisterFile("aaa/aaa.proto", fileDescriptor0) }
+
+var fileDescriptor0 = []byte{
+	// 172 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0x2c, 0x8e, 0x31, 0x0b, 0xc2, 0x30,
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+	0x0b, 0xfa, 0x40, 0x20, 0x68, 0xf1, 0x0d, 0x3e, 0x10, 0x4e, 0x76, 0x1a, 0x46, 0x47, 0x68, 0xe6,
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+	0x00, 0x00, 0xff, 0xff, 0x93, 0x24, 0x48, 0x9c, 0xbd, 0x00, 0x00, 0x00,
+}