Issue #2267 — Define semantics of file-path special variables when the
Taskfile is loaded from a remote source (HTTP/HTTPS/Git):
- TASKFILE / ROOT_TASKFILE: raw URL (fixes the broken `https:/...`
caused by filepath.Join collapsing the double slash)
- TASKFILE_DIR / ROOT_DIR: empty string — a DIR variable cannot point
to a URL
- TASK_DIR: resolved against USER_WORKING_DIR
Export taskfile.IsRemoteEntrypoint so the compiler can dispatch on the
nature of the entrypoint without relying on `c.Dir == ""` (a side
effect of the remote path).
