Allow loading extensions through python entry-points (#42370)

This PR adds the ability to load spack extensions through `importlib.metadata` entry 
points, in addition to the regular configuration variable.

It requires Python 3.8 or greater to be properly supported.

lib/spack/docs/configuration.rst

@@ -73,9 +73,12 @@ are six configuration scopes. From lowest to highest:
    Spack instance per project) or for site-wide settings on a multi-user
    machine (e.g., for a common Spack instance).
 
+#. **plugin**: Read from a Python project's entry points. Settings here affect
+   all instances of Spack running with the same Python installation.  This scope takes higher precedence than site, system, and default scopes.
+
 #. **user**: Stored in the home directory: ``~/.spack/``. These settings
    affect all instances of Spack and take higher precedence than site,
-   system, or defaults scopes.
+   system, plugin, or defaults scopes.
 
 #. **custom**: Stored in a custom directory specified by ``--config-scope``.
    If multiple scopes are listed on the command line, they are ordered
@@ -196,6 +199,45 @@ with MPICH. You can create different configuration scopes for use with
                mpi: [mpich]
 
 
+.. _plugin-scopes:
+
+^^^^^^^^^^^^^
+Plugin scopes
+^^^^^^^^^^^^^
+
+.. note::
+   Python version >= 3.8 is required to enable plugin configuration.
+
+Spack can be made aware of configuration scopes that are installed as part of a python package.  To do so, register a function that returns the scope's path to the ``"spack.config"`` entry point.  Consider the Python package ``my_package`` that includes Spack configurations:
+
+.. code-block:: console
+
+  my-package/
+  ├── src
+  │   ├── my_package
+  │   │   ├── __init__.py
+  │   │   └── spack/
+  │   │   │   └── config.yaml
+  └── pyproject.toml
+
+adding the following to ``my_package``'s ``pyproject.toml`` will make ``my_package``'s ``spack/`` configurations visible to Spack when ``my_package`` is installed:
+
+.. code-block:: toml
+
+   [project.entry_points."spack.config"]
+   my_package = "my_package:get_config_path"
+
+The function ``my_package.get_extension_path`` in ``my_package/__init__.py`` might look like
+
+.. code-block:: python
+
+   import importlib.resources
+
+   def get_config_path():
+       dirname = importlib.resources.files("my_package").joinpath("spack")
+       if dirname.exists():
+           return str(dirname)
+
 .. _platform-scopes:
 
 ------------------------

lib/spack/docs/extensions.rst

@@ -111,3 +111,39 @@ The corresponding unit tests can be run giving the appropriate options to ``spac
 
    (5 durations < 0.005s hidden.  Use -vv to show these durations.)
    =========================================== 5 passed in 5.06s ============================================
+
+---------------------------------------
+Registering Extensions via Entry Points
+---------------------------------------
+
+.. note::
+   Python version >= 3.8 is required to register extensions via entry points.
+
+Spack can be made aware of extensions that are installed as part of a python package.  To do so, register a function that returns the extension path, or paths, to the ``"spack.extensions"`` entry point.  Consider the Python package ``my_package`` that includes a Spack extension:
+
+.. code-block:: console
+
+  my-package/
+  ├── src
+  │   ├── my_package
+  │   │   └── __init__.py
+  │   └── spack-scripting/  # the spack extensions
+  └── pyproject.toml
+
+adding the following to ``my_package``'s ``pyproject.toml`` will make the ``spack-scripting`` extension visible to Spack when ``my_package`` is installed:
+
+.. code-block:: toml
+
+   [project.entry_points."spack.extenions"]
+   my_package = "my_package:get_extension_path"
+
+The function ``my_package.get_extension_path`` in ``my_package/__init__.py`` might look like
+
+.. code-block:: python
+
+   import importlib.resources
+
+   def get_extension_path():
+       dirname = importlib.resources.files("my_package").joinpath("spack-scripting")
+       if dirname.exists():
+           return str(dirname)