Use URL(fileURLWithPath:isDirectory) to avoid file system call

[marcelofabri]

Probably not going to make a huge impact, but I'll take anything that can make SwiftLint faster

https://twitter.com/bwebster/status/1111035640223678464

[SwiftLintBot]

	1 Warning
⚠️	Please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

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📖	Linting Aerial with this PR took 0.98s vs 0.98s on master (0% slower)
📖	Linting Alamofire with this PR took 1.1s vs 1.1s on master (0% slower)
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Here's an example of your CHANGELOG entry:

* Use URL(fileURLWithPath:isDirectory) to avoid file system call.  
  [marcelofabri](https://github.com/marcelofabri)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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