BIDS session path not resolved when subject has only one session

[surchs]

For subjects with at least one session, we rely on layout.get(..., return_type="dir") to get the path of a given session:

bagel-cli/bagel/bids_utils.py

Lines 72 to 79 in b6d15e9

	session_path = Path(
	layout.get(
	subject=bids_sub_id,
	session=session,
	target="session",
	return_type="dir",
	)[0]
	)

However, this fails if the subject has exactly one session.

To reproduce:

import bids
layout = bids.BIDSLayout("bids-examples/ieeg_motorMiller2007")
layout.get(subject="cc", session="01", target="session", return_type="dir")
Out[]: []

# BUT:
layout.get_sessions(subject="cc")
Out[]: ['01']

This is very similar to bids-standard/pybids#978.

We should do two things:

1. stop using return_type="dir" and replace it with building the path directly (as suggested in the issue)
2. being good neighbours and going over to pybids and see if we can help fix this return_type="dir" issue, e.g. by deprecating it / providing docs.