new reusable functions for using PathConfig

[jurgenvinju]

• this branch is a pure extension of another cleaner PR branch: clean-factor-path-config
• this branch adds basic functionality to compute file names from module names and module names from file names
• a reusable function for computing the latest relevant source file or binary file or library file based on the PathConfig and the timestamps of the relevant files
• the parameters for the above functions are computed from a LanguageFileConfig constructor, that explain which extensions, prefixes and filename escapes to use.