convert ltx:graphics candidates to file URIs

[xworld21]

Fix #2355 (allegedly) by making candidates a comma-separated list of URIs instead of plain file names.

I do not have conclusive proof about Windows, because the relevant tests seem to require a minimum texlive version, and I am using MikTeX. This would explain why CI did not catch the issue.

I suggest taking this PR further: make candidates a space separated list (since now spaces are %-encoded), and add this restriction to the schema (with something like list(anyURI)).

[brucemiller]

Interesting; basically you're trying to establish a consistent filename format that avoids OS dependencies. But rather than scattering lots of URI to/from around, and only dealing with @candidates at that, perhaps there's a common pinch point where the format can be made more consistent? Hmm....