This fixes an issue were "Find in Files" would not use the up-to-date
content of a file with unsaved changes.
The issue existed because 'FindInFilesWidget' uses
Project::for_each_text_file, which retrieves files by their absolute
path. However, when a file is opened in an Editor, it is created with
a relative path.
This caused us to store two ProjectFile objects that refer to the same
file - one with a relative path and one with an absolute path.
