Should be no absolute path names

[yarikoptic]

BIDS spec says

src/common-principles.md-Several metadata fields in BIDS require the specification of paths,
src/common-principles.md-that is, a string of characters used to uniquely identify a location in a directory structure.
src/common-principles.md-For example the `IntendedFor` or `AssociatedEmptyroom` metadata fields.
src/common-principles.md-
src/common-principles.md-Throughout BIDS all such paths MUST be specified using the slash character (`/`),
src/common-principles.md-regardless of the operating system that a particular dataset is curated on or used on.
src/common-principles.md-
src/common-principles.md:Paths SHOULD NOT be absolute local paths,
src/common-principles.md-because these might break when a dataset is used on a different machine.
src/common-principles.md-It is RECOMMENDED that all paths specified in a BIDS dataset are relative paths,
src/common-principles.md-as specified in the respective descriptions of metadata fields that require the use of paths.

good news is that there is only a single absolute path and I was just lucky to run into it:

❯ git grep 'Sources.*"/'
motion_correction_derivatives/sub-01_ses-baseline_desc-mc_pet.json:    "Sources": "/Users/martinnorgaard/Documents/GitHub/petprep_hmc/data/sub-01/ses-baseline/pet/sub-01_ses-baseline_pet.nii.gz",