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 `-`.
2b4f2daa73
