Workspace.from_url imports all .py files when creating LocalWorkspace

### 🐛 Describe the bug

When using Workspace.from_url() to create a local workspace, all other python files in the directory will be imported in the process. As a result, the code (outside of an `if __main__` block) in those scripts will be run.

Here's an example:

Directory structure:
./tango_workspace/
./exp.py
./other_file.py

**exp.py:**
```
from tango import step
from tango.workspace import Workspace

@step(cacheable=True, deterministic=True)
def test_step():
return 19

if __name__=='__main__':
ws = Workspace.from_url('local://tango_workspace')
print('ws', ws)
print('result', test_step().result())
```

**other_file.py:**
```
#some unrelated script that does stuff
print("hello world")
```

running `python exp.py` will try to create the workspace, but in the process will import other_file.py, causing it to run and output "hello world". This is obviously a big problem if there are other python scripts in the directory that shouldn't be run.

There are a few workarounds. I believe the issue stems from [this line](https://github.com/allenai/tango/blob/26a162ab133b07bf34aff21d48a80c484e5b04e3/tango/workspace.py#L165C13-L165C13) in the `cls.by_name` call. I haven't tried to fully understand this code, but I think since LocalWorkspace is not in the registry, it causes this behavior. Therefore some workarounds that solve this issue are to import LocalWorkspace somewhere before creating the workspace:

In exp.py:
```from tango import step
from tango.workspace import Workspace
from tango.workspaces.local_workspace import LocalWorkspace
...
```

But a more permanent solution is to add:
`from .workspaces import LocalWorkspace`
to the bottom of `tango/__init__.py` but I don't know if this is the best way to address this problem.

I am using version 1.2.1 on python 3.8.17. I think this issue would persist on 1.3 though

### Versions

Python 3.8.17
ai2-tango==1.2.1