Incorrect parsing of fig.path

[J-Moravec]

fig.path has quite specific undocumented requirements and can fail in corner cases.

MRE:

litedown::reactor(fig.path = "foo")
litedown::fuse(text = "```{r} \n plot(1) \n```")

File '.foochunk-1-1.png' not found (hence cannot be embedded).

1. foochunk-1-1.png is created
2. the path correction in fuse fails and appends a dot for a relative-path (instead of e.g., ./)
3. fig.path is not actually a path, but a path with prefix, e.g.,
4. the requirement of "/" is not documented

When fig.path contains /, it works.

litedown::reactor(fig.path = "foo/bar")
litedown::fuse(text = "```{r} \n plot(1) \n```", output = "markdown")

produces ![](<foo/barchunk-1-1.png>)

The problem likely lies in:

litedown/R/fuse.R

Lines 480 to 493 in 7846749

	set_path = function(name) {
	# fig.path = output__files/ if `output` is a path, otherwise use
	# litedown__files/ (we don't use _files because of rstudio/rmarkdown#2550)
	if (is.null(p <- opts[[name]])) p = aux_path(output_base %||% 'litedown', name)
	slash = endsWith(p, '/')
	# make sure path is absolute so it will be immune to setwd() (in code chunks)
	if (is_rel_path(p)) {
	p = file.path(getwd(), p)
	# preserve trailing slash because file.path() removes it on Windows
	if (slash) p = sub('/*$', '/', p)
	}
	opts[[name]] = p
	}
	set_path('fig.path'); set_path('cache.path')

IMHO, it feels like uneccesary complex code for just setting path, which is further complicated by it being an inner function.
is_rel_path itself has several layers of indirection.

Suggestion:

1. Make set_path a global function instead of an inner function, and make it a true function (if possible), not depending on local objects
2. make test for desired behaviour (there seems to be some history that is not documented with tests)

Discussion:

Is fig.path serving as a path prefix (instead of directory itself) a desired behaviour? It allows some nice things, but it making the behaviour quite a bit complicated from programming and user perspective (such as, the undocumented requirement for "/" if fig.path is really a path.

If you are happy about this suggestion, I will implement it.

[J-Moravec]

Fixed by #94, the issue wasn't actually in the set_path, but later when the a path is derived from fig.path in fuse_code.