Adds support for Spack package repositories from external git repos.

```yaml
repos:
  my_repo:
    git: https://example.com/example/example.git
    destination: ~/example  # optional
    paths:  # optional
    - subdir/spack_repo/example/x
    - subdir/spack_repo/example/y
```

If `destination` is not configured, Spack clones the git repo to `~/.spack/git_repos/{hash(repository)}`.

Package repositories can put a file `spack-repo-index.yaml` in their root with relative paths to roots of package repositories (i.e. directories that contain `repo.yaml`):

```yaml
repo_index:
  paths:
  - subdir/spack_repo/example/x
  - subdir/spack_repo/example/y
```

so users don't have to put that under `paths` in config. The `spack-repo-index.yaml` is simply a list of paths under `repo_index`. The idea is to avoid duplicating data such as "namespace" already specified in `<git repo>/<repo path>/repo.yaml`, to avoid that there are two sources of truth that go out of sync.

Further, `paths` in user config takes precedence, which allows users to enable specific package repositories in case the git monorepo provides multiple.

Remote package repositories are cloned/initialized in:
* `spack.main`
* `spack repo add`

This is process safe due to the lock in `$SPACK_USER_CACHE_PATH/package-repository.lock`; only one process can clone at a time.

The `spack repo add` command has a few new flags and a new positional arg:

```
spack repo add [-h] [--name NAME] [--path PATH] [--scope ...] path_or_repo [destination]
```

The signature is similar to the familiar `git clone <repository> <directory>`.

The `path_or_repo` argument is detected as a remote git repo if it contains a `:` not preceded by a `/`, which is what git does as well. If in the future we would support package repositories other than local file paths and remote git repos, we can resolve ambiguities with a new flag `[--git | --path | --<other-type>]`, but this is currently unnecessary.

The positional `destination` argument allows users to pick their own clone path, and only applies in case of git repos.

The flag `--path` corresponds to `repos:<name>:paths` and can be repeated to select specific package repositories in a git monorepo, and is also required if the git repo does not provide a `spack-repo-index.yaml` file in its root. Spack will never scan for `repo.yaml` files recursively, it only relies on `spack-repo-index.yaml` and `--repo-path` for package repository roots inside a git repo.

The flag `--name` applies to the *config* name/key in `repos.yaml` under `repos:<name>`. This flag is optional in the common case of adding just one package repository: it is set to the package repository's namespace. In case of monorepos with multiple package repositories, the `--name` flag is required.