fix: HTTPNode.Location when building graph

[jeongukjae]

After this commit, building internal graph can raise error because HTTPNode's Location value can be changed while building graph. (See line 65 (node.URL = url))

So if taskfile imports remote file with multiple depth (e.g. A --> remote B --> remote C) without filename, it raises

source vertex https://{PATH_TO_REMOTE}/Taskfile.yaml: vertex not found

So fixing HTTPNode.Location not to be changed over time.

(I couldn't find any related issues, so please let me know if there's issue or I have to create it)

[vmaerten]

lgtm! Thanks

Indeed, when no filename is provided, the node.URL is inconsistent, which causes issues with the graph.