Paths in `<PackageActionExtensions>` should be relative to the `--target` directory, and not the temp install dir

Often you'd want to run a cross platform 'tap' command as a PackageActionExtension, but currently there is a check for File.Exists which is not the case on windows for entry 'tap'. Instead, if the ExeFile specified in the action step is relative, we should always assume that the user wants to execute that action in the target directory.

The example below would currently run in temp OpenTAP directory, which I can't find a reasonable use-case where anyone would want that.

...
  </Files>
    <PackageActionExtensions>
    <ActionStep ActionName="install" ExeFile="tap" Arguments="remote configfile"/>
  </PackageActionExtensions>
</Package>