[Tizen] Backendlib should differ between xpk and wgt extensions

This commit fixes the way backend library pkgmgr's plugin will display type of application.
Extra query to pkgmgr is added to check for type of package - xpk or wgt.

Verify:
 - install xpk package,
 - install wgt package,
 - run: pkgcmd -l -q                           #xpk and wgt is listed
 - run: pkgcmd -l -q -t wgt                    #wgt is listed
 - run: pkgcmd -l -q -t xpk                    #xpk is listed
 - run: pkgcmd -q -s -n ${wgt_pkg_id} -t wgt   #wgt package is shown with wgt type in data
 - run: pkgcmd -q -s -n ${wgt_pkg_id} -t xpk   #no package should be found
 - run: pkgcmd -q -s -n ${xpk_pkg_id} -t wgt   #no package should be found
 - run: pkgcmd -q -s -n ${xpk_pkg_id} -t xpk   #xpk package is shown with xpk type in data
 - run: pkgcmd -q -s -p ${xpk_pkg_path} -t xpk #details should be listed
 - run: pkgcmd -q -s -p ${wgt_pkg_path} -t wgt #details should be listed

BUG=XWALK-2771
why not simply
return set_->pkg_type ?