Add version support for the Black action pt.2

Since we're moving to a composite based action, quite a few changes
were made. 1) Support was added for all OSes (Windows was painful).
2) Isolation from the rest of the workflow had to be done manually
with a virtual environment.

Other noteworthy changes:

- Since we can just put all of our action's code in action.yml, I
  got rid of the action/ directory and embedded the entrypoint in
  the YAML file.
- Renamed `black_version` to `version` to better fit the existing
  input naming scheme.
- Added support for log groups, this makes our action's output a
  bit more fancy (I may or may have not added some debug output too)

action.yml

@@ -2,24 +2,88 @@ name: "Black"
 description: "The uncompromising Python code formatter."
 author: "Łukasz Langa and contributors to Black"
 inputs:
-  black_args:
-    description: "Black input arguments."
+  options:
+    description:
+      "Options passed to black. Use `black --help` to see available options. Default:
+      '--check'"
+    required: false
+    default: "--check --diff"
+  src:
+    description: "Source to run black. Default: '.'"
+    required: false
+    default: "."
+  version:
+    description: 'Python Version specifier (PEP440) - e.g. "==20.8b1"'
     required: false
     default: ""
-  black_version:
-    description: 'Version of black to use. E.g "==20.8b1"'
+  black_args:
+    description: "[DEPRECATED] Black input arguments."
     required: false
     default: ""
 branding:
   color: "black"
   icon: "check-circle"
+
 runs:
   using: composite
   steps:
-    - run: pip install --upgrade --no-cache-dir black${{inputs.black_version}}
+    - id: black-venv
+      run: |
+        echo "::group:: Create a virtual environment for Black"
+
+        if [ "$RUNNER_OS" == "Windows" ]; then
+          bindir="Scripts"
+        else
+          bindir="bin"
+        fi
+
+        # TODO: check if this line is necessary
+        actionpath='${{github.action_path}}'
+        envpath="${{ github.action_path }}/.black-venv"
+        python -m venv $envpath
+        # Please don't ask me questions on why this specific form is necessary so the
+        # output doesn't up mangled. I really don't know why, but I don't want to
+        # dump even more hours on trying to understand it. BUT I will blame Windows
+        # for wasting a few hours of my life. ::set-output:: is weird... - @ichard26
+        printf "%s" "$envpath/$bindir/"
+        printf "%s\n" "::set-output name=venv-bin::"
+
+        echo
+        echo "created virtual environment at '$envpath'"
+        echo "::endgroup::"
+      shell: bash
+
+    - run: |
+        echo "::group:: Install Black"
+
+        if [ "$RUNNER_OS" == "Windows" ]; then
+          suffix=".exe"
+        else
+          suffix=""
+        fi
+
+        if [ -z "${{ inputs.version }}" ]; then
+          pipcmd="python$suffix -m pip install black[colorama]"
+        else
+          pipcmd="python$suffix -m pip install black[colorama]==${{inputs.version}}"
+        fi
+
+        echo "running from virtual environment '$pipcmd'"
+        echo
+        ${{ steps.black-venv.outputs.venv-bin }}$pipcmd
+
+        echo "::endgroup::"
       shell: bash
-    - id: run-black
-      run:
-        ${{ github.action_path }}/action/entrypoint.sh ${{ inputs.configuration }} ${{
-        inputs.sortPaths }}
+
+    - run: |
+        echo "::group:: Run Black"
+
+        if [ -n "${{ inputs.black_args }}" ]; then
+          echo '::warning::Input `with.black_args` is deprecated. Use `with.options` and `with.src` instead.'
+          ${{ steps.black-venv.outputs.venv-bin }}black ${{ inputs.black_args }}
+          echo "::endgroup::"
+        fi
+
+        ${{ steps.black-venv.outputs.venv-bin }}black ${{ inputs.options }} ${{ inputs.src }}
+        echo "::endgroup::"
       shell: bash