Extract the logic for resolving files

QDoc has the need to find files during the processing of a documentation set based on its current configuration.

For example, when looking for a file to quote from.

Currently, QDoc exposes the logic for this as part of the Config class, which use is later centralized into the Doc which is depended upon by other parts of the system, such as DocParser or WebXMLGenerator.

Logically, QDoc performs a simple linear search over a series of config-defined directories and config-defined known-files.

This simple logic should be extracted from Config, moved into its own system with no dependencies on other parts of QDoc and then used instead of the centralized Doc dependency.

Some dead code was identified in the parts relevant to this processing, such as the use of the userFriendlyFilePath argument that permeates the searches for files. That code should be removed in the process.

Similarly, some footguns were indentified in the way in which QDoc processes files, such that the result of a search may be silently changed, in very specific cases, when adding to the list of known directories, even if the search is not related to the configuration change itself.

Furthermore, repeated safety-checks or possibly missing safety checks were identified in the same code, such that the new system should provide a simple interface that guarantees where and how checks are performed.

The extracted system should be tested in full and then inserted back into the codebase, subsidizing each use of file searching in the code base.