Porting away from NetAccess, step 1: code refactoring

Summary:

• Split out the NetAccess::exists call from isFromExistingURL(bool),

only one caller was passing true (the default value). Simpler to
remove that bool and call both methods in that one location.

• Provide a urlExists() method to wrap the calls to NetAccess::exists

and call it from as many places as possible.

Test Plan:
Builds, I can open and save a file, but it would take time to figure
out how to trigger all the changed code paths.

Reviewers: broulik, vkrause

Reviewed By: vkrause

Maniphest Tasks: T11576

Differential Revision: https://phabricator.kde.org/D25468