Capture package metadata before install start

Summary:
KPackage (despite being supposedly internally shared) is not kept as a
strong reference inside PackageJob. Potentially it could go out of scope
before a job ends.

The signals to emit changes relied on the package. This patch captures
relevant metadata at the start of the install.

noCrashOnAsyncInstall test now passes again

Test Plan: Ran test

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16733

(cherry picked from commit 451b1785e8d0de9f656235baad1eaf988ebc1d5a)