[WIP] Fix relative references when store not in cwd

[rly]

Motivation

Fix #255. Slight rework of how references are handled.

Currently, the "source" of links and references are relative to the directory that contains the currently open Zarr store.

Internal Zarr links and references use the name of the Zarr store as the "source". For example, if file1.zarr contains an internal link, that link looks like:

{
    "name": "foo_link",
    "path": "/buckets/bucket1/foo_holder/foo1",
    "source": "file1.zarr"
}

External links: If file0.zarr and file1.zarr are in the same directory, and file1.zarr has an external link to a group in file0.zarr, that link looks like:

{
    "name": "foo_link",
    "path": "/buckets/bucket1/foo_holder/foo1",
    "source": "file0.zarr"
}

When reading file1.zarr from elsewhere, e.g., ZarrIO("a/b/c/file1.zarr", mode="r"), we need to make sure that the relative paths are resolved correctly from a/b/c. Approach 1: Fix this.

An alternative approach, which I believe was the original implementation, is that instead of writing paths relative to the directory that contain the currently open Zarr store, write paths relative to the currently open Zarr store. So internal links and references would have the source: "." and the above external link example would have the source: "../file0.zarr". Approach 2: Return to this pre-0.10.0 way of storing "source" in links and references.

I propose we take approach 2, because otherwise links in files written under the original implementation would not be readable - I tested this and indeed files with links written in 0.8.0 cannot be read in 0.10.0 - and approach 2 does not suffer any disadvantages in my eyes.

@mavaylon1 Let me know what you think.

TODO

• Update tests once we settle on an approach

Checklist

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