ensure entrypoint parent folder is in the module search path (#374)

[This SO answer](https://stackoverflow.com/a/62923810) provides good
detail about the difference between launching a python file (aka `python
app/main.py`) and launching a python module (aka `python -m
widget_store.main`). In particular for this PR, launching a file
automatically adds the file's parent directory to `sys.path` while
launching a module adds the current directory.

`dbos debug` was always ensuring the current directory (represented by
empty string) was in `sys.path`, so it was failing when attempting to
debug a file in a subdirectory. This code fixes that + isolates the
change to `sys.path` to only happen for file launch. Module launch does
not require `sys.path` changes because `dbos debug` is already launching
in the debugger as a module so the current directory will already be on
the path