Allow .dev package versions to have relative paths

[rushmobius]

Currently if using a local repo and I add a haxelib dev target it always ends up being an absolute path. For a single developer this isn't a big deal, but on a multi-user team, it would be nice to use relative paths.

Given the following

C:\MyProject\lib1...
C:\MyProject.haxelib

If I run haxelib dev lib1 lib1 from C:\MyProject, the .dev file that gets created is C:\MyProject\lib1 instead of being relative to the repository root