* Bump the package API of the `builtin` repo to `v2.0`
* Move `var/spack/repos/builtin` -> `var/spack/repos/spack_repo/builtin`
* Move test repos `var/spack/repos/{builtin.mock,tutorial,...}` -> `var/spack/test_repos/`
* Update package dir names to v2 format (`-` -> `_` etc)
* Change absolute imports `from spack.pkg.builtin.my_pkg ...` to relative imports `from ..my_pkg.package ...`
Users who have a repo on top of builtin should change imports from
```python
from spack.pkg.builtin.my_pkg import MyPkg
```
to
```python
from spack_repo.builtin.packages.my_pkg.package import MyPkg
```
and can configure their editors with
```
PYTHONPATH=$spack/lib/spack:$spack/var/spack/repos
```
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