If you used add with dev + a relative path an absolute path ends up on Manifest.toml / meta.json

If i do something like

```python
juliapkg.add("PowerfulCore", dev=True, , path="../../julia/PowerfulCore/")
```

to add a julia dependency to a python package within a monorepo then it installs and works correctly.
But the `Manifest.toml` / `meta.json` end up with a relative path in it.

This is technically fine as when i checkout the repo in a new location it just updates to the new path.
But it means that if I version control this file it has a lot of meaningless changes.

`Manifest.toml`'s do support relative paths.