package API v2.0: new repo layout (#49256)

This implements Package API v2.0, and is an opt-in feature for repos. It can be enabled with

```yaml
repo:
  ...
  api: v2.0
```

It differs from the current default v1.0 as follows:

1. Package names can only contain `-` as a separator.
2. Package names can only be lowercase.
3. Package directory names are valid Python module names.
4. The repo namespace and its directory name are the same.
5. The `packages` subdir, which is configurable, should be a directory
   name that is also a valid Python module name.
6. There is a one to one mapping between Spack package names and Python
   module names.
7. Import statements `import spack.pkg.namespace.package_module` in
   `package.py` files need to specify the canonical package module.

To go from Spack package name to Python module name:
- Replace `-` by `_`
- Add a leading `_` if the package name starts with a digit

To go from Python module name to Spack package name:
- Strip leading `_`
- Replace `_` by `-`.

lib/spack/docs/developer_guide.rst

@@ -154,9 +154,7 @@ Package-related modules
 
 :mod:`spack.util.naming`
   Contains functions for mapping between Spack package names,
-  Python module names, and Python class names. Functions like
-  :func:`~spack.util.naming.mod_to_class` handle mapping package
-  module names to class names.
+  Python module names, and Python class names.
 
 :mod:`spack.directives`
   *Directives* are functions that can be called inside a package definition