win32: improve the package installation dir lookup
win32: improve the package installation dir lookup

As an example, the core of gedit is in a private library
placed in %INSTALLDIR%/lib/gedit/libgedit.dll

Before this patch we would get %INSTALLDIR%/lib/gedit as the
installation package dir, while what we actually want is to get
%INSTALLDIR%

https://bugzilla.gnome.org/show_bug.cgi?id=733934

johne53/MB3Glib,MathieuDuponchelle/glib,lukasz-skalski/glib,mzabaluev/glib,lukasz-skalski/glib,mzabaluev/glib,lukasz-skalski/glib,MathieuDuponchelle/glib,endlessm/glib,MathieuDuponchelle/glib,tchakabam/glib,johne53/MB3Glib,cention-sany/glib,ieei/glib,tchakabam/glib,tamaskenez/glib,krichter722/glib,johne53/MB3Glib,mzabaluev/glib,gale320/glib,tchakabam/glib,krichter722/glib,Distrotech/glib,ieei/glib,Distrotech/glib,cention-sany/glib,gale320/glib,krichter722/glib,cention-sany/glib,johne53/MB3Glib,ieei/glib,gale320/glib,johne53/MB3Glib,MathieuDuponchelle/glib,tamaskenez/glib,lukasz-skalski/glib,endlessm/glib,endlessm/glib,Distrotech/glib,gale320/glib,cention-sany/glib,Distrotech/glib,mzabaluev/glib,lukasz-skalski/glib,endlessm/glib,endlessm/glib,tamaskenez/glib,tchakabam/glib,MathieuDuponchelle/glib,cention-sany/glib,krichter722/glib,Distrotech/glib,gale320/glib,mzabaluev/glib,ieei/glib,ieei/glib,tamaskenez/glib,tamaskenez/glib,tchakabam/glib,johne53/MB3Glib,krichter722/glib