Warn users if they create a package in a workspace without listing them as a member

cargo warns users with a warning if they create a package inside a workspace directory like the following:

```console
warning: compiling this new package may not work due to invalid workspace configuration

current package believes it's in a workspace when it's not:
current: /Users/kayagokalp/fuel/dev/sway/forc-error/Cargo.toml
workspace: /Users/kayagokalp/fuel/dev/sway/Cargo.toml
```

We might want to do something similar. In our case a child package wouldn't be "thinking" it's in a workspace but it is still pretty easy to forget listing a package in the workspace manifest file.