Feature tool-cache for Gleam and rebar3 (#223)

* Make iterating over mirrors more flexible (and reusable)

At the same time we fix an error, where we aren't using the
mirrors for the OTP build :)

* Make installation of cache-based tool callback -based

This hopefully eases introduction of new languages at the cost
of a bit of abstraction (there's an extensive comment on how to
fill in the install options' object)

We'll soon test this abstraction by making Gleam and rebar3
cached too

* Increase our visibility when debugging

* Improve on lessons learned to ease incorporation of further caches

(starting with Gleam on Linux)

* Cache Gleam on Windows

* Cache Rebar3 on Linux

Also, gets rid of all .sh

* Cache Rebar3 on Windows

Also, gets rid of all .ps1

* Act on CI results

And also be consistent when it comes to using
fs....Sync vs fs.promise....

* Move tests to a single file

* Move main executable to a single file

* Decrease number of changes to ease review

* Expose function required for tests

* Don't async/await when not required

* Resolve post- merge-conflict resolution issues

package.json

@@ -8,8 +8,8 @@
     "jslint": "eslint src/**/*.js && eslint test/**/*.js",
     "licenses": "yarn licenses generate-disclaimer > 3RD_PARTY_LICENSES",
     "markdownlint": "markdownlint *.md ./github/**/*.md",
-    "shellcheck": "shellcheck src/install-*.sh .github/workflows/*.sh",
-    "test": "node test/setup-beam.test.js && node ./test/problem-matchers.test.js",
+    "shellcheck": "shellcheck .github/workflows/*.sh",
+    "test": "node test/setup-beam.test.js",
     "yamllint": "yamllint .github/workflows/**.yml && yamllint .*.yml && yamllint *.yml",
     "build-dist": "npm install && npm run build && npm run format && npm run markdownlint && npm run shellcheck && npm run yamllint && npm run jslint"
   },