Propagates imports instead of cloning dependent messages

[rafaparadela]

This PR closes #74

In summary:

If we have author.proto:

package com.acme;

message Author {
    string name = 1;
    string nick = 2;
}

and a book.proto:

package com.acme;

import "author.proto";

message Book {
    int64 isbn = 1;
    string title = 2;
    repeated Author author = 3;
}

If we try to produce the scala code given the book.proto

What we had previously

We are generating code like this:

package com.acme

object book {
  @message final case class Author(name: String, nick: String)
  @message final case class Book(isbn: Long, title: String, author: List[Author])
}

What we have now

We are generating code like this:

package com.acme

import com.acme.author.Author

object book {
  @message final case class Book(isbn: Long, title: String, author: List[Author])
}

[codecov]

Codecov Report

Merging #75 into master will increase coverage by 0.61%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   53.12%   53.73%   +0.61%     
==========================================
  Files          17       17              
  Lines         753      763      +10     
  Branches        4        3       -1     
==========================================
+ Hits          400      410      +10     
  Misses        353      353

Impacted Files	Coverage Δ
...ain/scala/higherkindness/skeuomorph/mu/print.scala	82.6% <100%> (+1.96%)	⬆️
...igherkindness/skeuomorph/protobuf/ParseProto.scala	89.04% <100%> (ø)	⬆️
.../scala/higherkindness/skeuomorph/mu/protocol.scala	52% <60%> (+2%)	⬆️
...la/higherkindness/skeuomorph/protobuf/schema.scala	90.78% <0%> (+1.31%)	⬆️

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