Support git dependency subfolder option

[elsassph]

haxelib git allows (though it isn't documented) to specify a path inside the repo to look up for the library instead of considering it to be at the root:

Example: https://github.com/massiveinteractive/MassiveUnit/tree/master/src

haxelib git munit [email protected]:massiveinteractive/MassiveUnit.git 2.3.4 src

Potentially we don't need the sub-folder to be configurable but maybe lix could look up src/haxelib.json automatically, as it seems it automatically looks up src as classpath.