support multi contract in a file

[nakajo2011]

problem contract file example:

contract A {
  function hoge() public returns(uint) {
  return 1;
  }
}

contract B {
  function fuga() public returns(uint) {
  return 2;
  }
}

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Like this. can not capturing correct filename if multi contract define in a file.

[nakajo2011]

artifact json files is separated, but file index is not incremental in sourceMap.
Solution idea bellow:

1. mapping key change to what filename:contractName instead of filename only.
2. update finding process that artifact info from bytecode, and update mapping process of filename.