Fix assert due to non normalized indexed string

Summary:
Assert in KDevelop::BackgroundParser::trackerForUrl(KDevelop::IndexedString const&)
ensures that indexed strings are normalized. Thus use normalized
url for creating indexed string. Also add assert to find origins of possible
further uses of unnormalized urls for constructing normalized strings.

Reviewers: brauch, KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2312