Change primary import location to `idaes_flowsheet_processor`

At the moment, the primary import location is `idaes_flowsheet_processor.api`, i.e. we have:

```py
from idaes_flowsheet_processor.api import FlowsheetInterface
```

However, I think this is mostly a remnant of the original package structure, and we could just get rid the `api` subpackage, i.e.:

```py
from idaes_flowsheet_processor import FlowsheetInterface
```
