When a specific haxelib version is requested, ensure that that version is included.

Sometimes libraries request to include a specific version of a dependency, e.g. HaxePunk includes OpenFL version 1.2.3 so that changes in the latest version of OpenFL won't cause HaxePunk to break. However, if any other dependencies also include OpenFL, the latest version will be included, causing a conflict.

This pull request changes include.xml parsing so that (1) only one version of any given library is ever included in a project, and (2) if a specific version of a library is requested in one file, that version will be the one that is included, so if two different dependencies contain

    <haxelib name="openfl" version="1.2.3" />

and

    <haxelib name="openfl" />

1.2.3 will always be the one that's used.