Parse all schemas as one Document

[joffrey-bion]

This makes use of the MultiSourceReader provided by the graphQL library.
Using this reader allows to read a single document from multiple files,
while correctly associating line numbers and errors to their original
source file.

This doesn't bring any overhead because the method Parser.parseDocument(String) that was previously used already created a MultiSourceReader itself.
Note that the previous method didn't associate the source file name with the content used for parsing, while the new one does.

It allows to solve cross-reference problems with union types for instance.

Resolves:
#27

[kobylynskyi]

@joffrey-bion some tests are failing. Could you please take a look?

[joffrey-bion]

Yes, I'm fixing the tests.
It's because of the whitespace at the end of some test files.

I'll actually make a second PR for the removal of the trim() and the cleanup of the tests.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #28      +/-   ##
============================================
+ Coverage     87.16%   87.33%   +0.16%     
- Complexity      217      220       +3     
============================================
  Files            29       29              
  Lines           522      529       +7     
  Branches         73       74       +1     
============================================
+ Hits            455      462       +7     
  Misses           44       44              
  Partials         23       23

Impacted Files	Coverage Δ	Complexity Δ
...om/kobylynskyi/graphql/codegen/GraphqlCodegen.java	86.58% <100%> (ø)	24 <0> (ø)	⬇️
...lynskyi/graphql/codegen/GraphqlDocumentParser.java	90.9% <100%> (+15.9%)	5 <4> (+3)	⬆️

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[joffrey-bion]

I've made a separate PR, please take a look: #29

This one is based on the 29, so the diff will get cleaner when the other one is merged.

[kobylynskyi]

Awesome, thank you. #29 was merged.
Will merge this one after the following change will be done:
#28 (comment)

[joffrey-bion]

I added the requested change. Please don't hesitate to add any other comment.

[kobylynskyi]

@joffrey-bion thank you so much for the contribution.
Will release the change in 1.4.2 today

[joffrey-bion]

@kobylynskyi Well, thanks to you for being so quick to review, merge and release!