Prevent babelrc package import failure on relative current path

[kwonoj]

Summary

Relates to danger/danger-js#261 .

This PR attempts to fix cases of reading babelrc via package.json. Using current code path, below case is possible:

//directory is '.', relative current
const packageJsonFilePath = path.join(directory, PACKAGE_JSON); 
//now packageJsonFilePath === PACKAGE_JSON, plain filename only as 'package.json'
//fs can check file exists
if (fs.existsSync(packageJsonFilePath)) { 
  //node module resolution failure cause it doesn't have paths, 
  //node try to resolve it as node_modules -> exception thrown
  const packageJsonFileContents = require(packageJsonFilePath);
...

I thought of using different mechanics to detect relative current (i.e path.relative(directory, '.'), path.resolve(directory, __dirname)..) but took simplest comparison instead.

Test plan
I've verified by replacing this module uses it as dependency (danger/danger-js#261)

[codecov-io]

Codecov Report

Merging #3723 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3723   +/-   ##
=======================================
  Coverage   62.64%   62.64%           
=======================================
  Files         183      183           
  Lines        6709     6709           
  Branches        6        6           
=======================================
  Hits         4203     4203           
  Misses       2503     2503           
  Partials        3        3

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[cpojer]

Thanks!

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