restore old behaviour of FindPulseAudio

Summary:
Commit ee5b036c added a new FindPulseAudio module with a slightly different syntax than the old one. Since the old module was removed with commit 75740228 plasma-desktop and plasma-pa are broken. Just adding the old syntax as synonym for fixing this.

See also https://bugs.kde.org/show_bug.cgi?id=386659

Test Plan: compile tested with plasma-desktop

Reviewers: #frameworks, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: jriddell, ngraham, rikmills, cgiboudeaux, #build_system

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D8777