Add option to use client side mapping for p4 change detection

Currently we will always look up the depot mapping to use for change detection because we can't guarentee the client stuff will work on the server (e.g. the plans may usually build on a remote agent). However, by doing this, we lose the ability to handle perforce overlay mappings. i.e.

//depot/stuff/...  //client/stuff/...
+ //depot/other/... //client/stuff/other/...

If we add an option to always use the client side of the mapping rather than doing the look up, we might be able to work around the limitations and customers who can use the client on the server can also use overlay mappings.