Added quoting of import paths

This fixes generation of import paths, otherwise VisualD's build will not work when paths contain spaces.

Example, before:
``` xml
<imppath>C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\vibe-d-0.7.18\source C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\libevent-master C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\openssl-master</imppath>
```
after:
``` xml
<imppath>"C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\vibe-d-0.7.18\source" "C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\libevent-master" "C:\Documents and Settings\Piotr\Dane aplikacji\dub\packages\openssl-master"</imppath>
```