Locates the POM file within the specified project directory. In case the given project directory does not exist
or does not contain a POM file, the return value indicates the expected path to the POM file. Sub directories of
the project directory will not be considered when locating the POM file. The return value will be an absolute
path if the project directory is given as an absolute path.
projectDirectory - The (possibly non-existent) base directory to locate the POM file in, must not be null.
The path to the (possibly non-existent) POM file, never null.