Respect specified haxelib server url even if dependency itself is installed from a non-haxelib source

[gene-pavlovsky]

Use case: a dependency is installed from github, however it's own dependencies are installed from haxelib.
If, for some reason, it's desired to use a private haxelib server (e.g. a caching proxy), it's currently impossible in the above use case.
lix install --haxelib-url haxelib-server gh:org/lib should get the org/lib library from github, but when resolving it's own dependencies, it should respect the haxelib-server specified with --haxelib-url.