refactor: packages import spack.package explicitly (#30404)

Explicitly import package utilities in all packages, and corresponding fallout.

This includes:

* rename `spack.package` to `spack.package_base`
* rename `spack.pkgkit` to `spack.package`
* update all packages in builtin, builtin_mock and tutorials to include `from spack.package import *`
* update spack style
  * ensure packages include the import
  * automatically add the new import and remove any/all imports of `spack` and `spack.pkgkit`
    from packages when using `--fix`
  * add support for type-checking packages with mypy when SPACK_MYPY_CHECK_PACKAGES
    is set in the environment
* fix all type checking errors in packages in spack upstream
* update spack create to include the new imports
* update spack repo to inject the new import, injection persists to allow for a deprecation period

Original message below:
 
As requested @adamjstewart, update all packages to use pkgkit.  I ended up using isort to do this,
so repro is easy:

```console
$ isort -a 'from spack.pkgkit import *' --rm 'spack' ./var/spack/repos/builtin/packages/*/package.py
$ spack style --fix
```

There were several line spacing fixups caused either by space manipulation in isort or by packages
that haven't been touched since we added requirements, but there are no functional changes in here.

* [x] add config to isort to make sure this is maintained going forward

lib/spack/docs/developer_guide.rst

@@ -151,7 +151,7 @@ Package-related modules
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 :mod:`spack.package`
-  Contains the :class:`~spack.package.Package` class, which
+  Contains the :class:`~spack.package_base.Package` class, which
   is the superclass for all packages in Spack.  Methods on ``Package``
   implement all phases of the :ref:`package lifecycle
   <package-lifecycle>` and manage the build process.

lib/spack/docs/packaging_guide.rst

@@ -2393,9 +2393,9 @@ Influence how dependents are built or run
 
 Spack provides a mechanism for dependencies to influence the
 environment of their dependents by overriding  the
-:meth:`setup_dependent_run_environment <spack.package.PackageBase.setup_dependent_run_environment>`
+:meth:`setup_dependent_run_environment <spack.package_base.PackageBase.setup_dependent_run_environment>`
 or the
-:meth:`setup_dependent_build_environment <spack.package.PackageBase.setup_dependent_build_environment>`
+:meth:`setup_dependent_build_environment <spack.package_base.PackageBase.setup_dependent_build_environment>`
 methods.
 The Qt package, for instance, uses this call:
 
@@ -2417,7 +2417,7 @@ will have the ``PYTHONPATH``, ``PYTHONHOME`` and ``PATH`` environment
 variables set appropriately before starting the installation. To make things
 even simpler the ``python setup.py`` command is also inserted into the module
 scope of dependents by overriding a third method called
-:meth:`setup_dependent_package <spack.package.PackageBase.setup_dependent_package>`
+:meth:`setup_dependent_package <spack.package_base.PackageBase.setup_dependent_package>`
 :
 
 .. literalinclude:: _spack_root/var/spack/repos/builtin/packages/python/package.py
@@ -3022,7 +3022,7 @@ The classes that are currently provided by Spack are:
 +----------------------------------------------------------+----------------------------------+
 |     **Base Class**                                       |           **Purpose**            |
 +==========================================================+==================================+
-| :class:`~spack.package.Package`                          | General base class not           |
+| :class:`~spack.package_base.Package`                     | General base class not           |
 |                                                          | specialized for any build system |
 +----------------------------------------------------------+----------------------------------+
 | :class:`~spack.build_systems.makefile.MakefilePackage`   | Specialized class for packages   |
@@ -3153,7 +3153,7 @@ for the install phase is:
     For those not used to Python instance methods, this is the
     package itself.  In this case it's an instance of ``Foo``, which
     extends ``Package``.  For API docs on Package objects, see
-    :py:class:`Package <spack.package.Package>`.
+    :py:class:`Package <spack.package_base.Package>`.
 
 ``spec``
     This is the concrete spec object created by Spack from an