Introduce rebar-version (#9)

Previously (Remove redundant code) (+27 squashed commits)
Squashed commits:
[0b79c31] Rename as setup-beam
[312b42b] Simplify platform verification
[b43ca96] Prevent use of OTP- prefixed Erlang/OTP versions
[103a236] Move the promise-based approach to an async-based approach

(not testing for failure, so not guaranteeing readable/usable output either)
[1e7f631] Remove local test files
[d03e0c6] Prevent use of -otp- infixed Elixir versions
[697110e] Have versions output in action, not just in tests
[7360781] Ease test validations
[71e6999] Reinstall tests that were there before
[6602f20] Attempt at simplifying regexp matching with build listings
[09247af] Provide for an Elixir to "not OTP" fallback mechanism

(as explained by the comments)
[dfc4083] Attempt at renaming for clarity

We also get rid of return values when not reachable
We also move the `-otp-` part of the Elixir version upstream so it's easy to reason about
  (and update the tests accordingly)
[0d4eb28] Bump our dep.s

(trying to figure out why core.setFailed isn't making the action fail, as it should)
[ebf48ef] Have action fail (with an exception) when no expected version is found

(we otherwise feel that a simple warning is not enough, as it may end up hiding potential bugs)
[8478364] Adapt tests to our actual inputs
[16ea63d] Add prettier again, now sync'ed with jslint

(pass it on the code, already)
[cfc65df] Improve error messages and tests
[a017948] Test unavailable Elixir upon mix
[7f2a3e9] Test failing installations

Also output core.error when expected
[08061ca] Attempt at making shell scripts more robust
[4ee73cb] Separate test code from production code

1. inputs (for CI) are managed by code, not by .yml env. variables
2. rebar3_builds.txt added for stable repeatability
3. tests tweaked to make it easier to modify and drop dependency on production code
4. `dist` regenerated (duplicate files gone)
5. setup-elixir.js tweaked to this new reality
[d17fed4] Remove prettier from the package
[7d6f8f5] Actually break CI when we should
[b972eaf] Remove unrequired element
[0ffa28a] Ease maintenance and readability
[4690169] Fix as per broken tests
[a1371c6] Merge setup-erlang in

test-projects/mix/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

test-projects/mix/.gitignore

@@ -0,0 +1,24 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+test-*.tar
+

test-projects/mix/README.md

@@ -0,0 +1,21 @@
+# Test
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `test` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:test, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at [https://hexdocs.pm/test](https://hexdocs.pm/test).
+

test-projects/mix/lib/test.ex

@@ -0,0 +1,18 @@
+defmodule Test do
+  @moduledoc """
+  Documentation for Test.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Test.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

test-projects/mix/mix.exs

@@ -0,0 +1,29 @@
+defmodule Test.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :test,
+      version: "0.1.0",
+      elixir: "~> 1.0",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      {:logfmt, "~> 3.3"}
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

test-projects/mix/mix.lock

@@ -0,0 +1,3 @@
+%{
+  "logfmt": {:hex, :logfmt, "3.3.0", "f863e19b9cac952f177f878074ec9304bb132eb9034306e75216dee88ca744d9", [:mix], [], "hexpm"},
+}

test-projects/mix/test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()

test-projects/mix/test/test_test.exs

@@ -0,0 +1,9 @@
+defmodule TestTest do
+  use ExUnit.Case
+  doctest Test
+
+  test "greets the world" do
+    out = Logfmt.encode(foo: "bar")
+    assert out == "foo=bar"
+  end
+end