install package with file: protocol as default if it exists in the filesystem

[voxsim]

This pull request should resolve #605.

Some info:

• I added existsSync function to not spread async annotation everywhere
• I created an instance method normalize that 'do the stuff', in detail:
  everytime we normalizePattern in package-request and it is a request we also normalize the range (if it is a file it will preprend file: and do other stuff, read the comment on the method).
• normalizePattern is a static function and it is used in other classes, I prefered to do not touch this method

Let me know if you like it!
cheers,
Simon

[bestander]

I think we should not introduce Sync functions unless there is a really strong reason.
How bad would it be to change the function to async?

Otherwise, LGTM.

[bestander]

The tests will pass when you rebase

[voxsim]

@bestander I will try to do the same with async in another pull request and after that we will decide which is the best :)

[bestander]

👍

[voxsim]

@bestander I rebased with master and I opened another pull request #2723 ;)

[bestander]

#2723 is better :)