Bzlmod: Support local_path in Bazel Registry's source.json file

[meteorcloudy]

Currently the source.json (e.g. for grpc) file in a Bazel registry only accepts attributes that can be translated to a http_archive repository rule, Bazel then delegates the fetching of the Bazel module to the http_archive rule.

But when users try to set up their own Bazel registry, they might want to point a Bazel module to a different path that hosts the source of the Bazel module (instead of an archive). Therefore, it would be useful to support other types of repositories in the source.json file, and an attribute 'type' to differentiate between them.

Supported types:
'archive' : the default value if no type was provided
'local_path'

E.g.

{
    "type": "local_path",
    "path": "/Users/foo/bar/project_x"
}

{
    "type": "local_path",
    "path": "../foo/bar/project_y"
}

For the second case where path is a relative path, we need to decide which path it should be relative to.
To solve this we can add a module_base_path in the bazel_registry.json file of the registry.

Then we resolve path by the following rules:

1. If path is already an absolute path, then just use it.
2. If path is a relative path and module_base_path is an absolute path, then path is resolved to <module_base_path>/<path>
3. If path and module_base_path are both relative paths, then path is resolved to <registry_path>/<module_base_path>/<path>.

Note that in the 3rd case, we need to parse registry_path from the registry URL, which means the registry must be hosted locally and used by --registry=file://<registry_path> (instead of https://). Otherwise, Bazel should throw an error.