Fix elixir 1.0.x compatibility

[mneudert]

After the last changes the backwards compatibility with Elixir 1.0.x versions has been accidentally broken.

Changing MapSet.new to the already existing Hex.Set.new wrapper should fix that.

This was the actual error message I encountered:

** (UndefinedFunctionError) undefined function: MapSet.new/1 (module MapSet is not available)
    MapSet.new(["CHANGES.md", "LICENSE", "mix.exs", "rebar.config", "README.md"])
    (hex) lib/hex/scm.ex:142: Hex.SCM.guess_build_tools/1
    (hex) lib/hex/scm.ex:111: Hex.SCM.checkout/1
    (mix) lib/mix/dep/fetcher.ex:64: Mix.Dep.Fetcher.do_fetch/3
    (mix) lib/mix/dep/converger.ex:154: Mix.Dep.Converger.all/8
    (mix) lib/mix/dep/converger.ex:163: Mix.Dep.Converger.all/8
    (mix) lib/mix/dep/converger.ex:47: Mix.Dep.Converger.converge/4
    (mix) lib/mix/dep/fetcher.ex:16: Mix.Dep.Fetcher.all/3

[ericmj]

Thank you @mneudert.

I think the regression tests missed this because this code is only hit for old packages and it's tricky for us to test packages that the server no longer accepts. Would you mind making the function public and adding a unit test for it?

[mneudert]

Does the test what you were looking for?

(It already catched the Enum.into/2 I missed as the old HashSet does not provide a new/1...)

[ericmj]

Hex.SCMTest :)

[mneudert]

No one has ever seen that 🙄

[ericmj]

Looks great. Will merge when CI passes. ❤️