Ensure all modules are properly loaded for "mix gettext.merge"

[c4710n]

:nofile error of #358 has been solved for a while, but I found it's not solved completely.

I can still see the :nofile error when working with Gettext with custom plural module. I created a repo for reproducing the problem:

• repo: https://github.com/plastic-gun/elixir-gettext-nofile
• every commit in this repo is self-descriptive, you can track them to reproduce the problem.
• README.md contains the following contents:
  • how to reproduce?
  • what is wrong?
  • the problem
  • the reason
  • the solution

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I found the solution is very easy - use mix loadpaths, such as:

$ mix do loadpaths + gettext.merge priv/gettext --locale en_US

But, to minimize surprises, I believe it would be better to make the changes within Mix.Tasks.Gettext.Merge, which helps to reduce the friction for other developers. This PR does it.

I don't have a comprehensive understanding of mix's workings yet, so the solution in this PR maybe not perfect. Welcome to comment. ;)

And, I want to create a test for this case, but I don't know how. Sorry for that :(

[coveralls]

Pull Request Test Coverage Report for Build e71d886bcb0d9276df8d467fb2025640484e0e36-PR-365

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 90.801%

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Totals
Change from base Build 8cc657d6704b420ffdb93c51b640bf9dc5d5614a:	0.06%
Covered Lines:	533
Relevant Lines:	587

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💛 - Coveralls

[maennchen]

@c4710n Thanks!

The PR looks good to me, but @whatyouhide knows more about the internals around module loading I presume. So we should await his feedback.

[c4710n]

@whatyouhide New change's done.

[whatyouhide]

Thanks @c4710n! 💟