Normalize paths to sub modules

When collecting the sub modules during a reactor build, the path to the module POMs should always be normalized. Currently, this happens only on a Windows platform via File.getCanonicalFile(). The attached patch adds normalization (but not canonicalization) for other platforms, too.

The motivation: Consider a multi module project with the following directory structure:
project/
project-parent/
project-module/
such that the parent POM in project-parent will contain
<module>../project-module</module>
to reference the sub module. Simple string/path concatenation will therefore deliver a path like

/project-module
instead is surely better, and may it be just for nice log output.

However, certain plugins/tools try to detect symlinks by comparing the canonicalized path with the absolute path of a file. While users of DirectoryScanner are usually fine because this class always canonicalizes the base directory before the check, code that does not know about a base directory but simply gets a single file will erroneously detect a symlink because ".." gets removed during canonicalization.

This actually happens with the CpdReport of the maven-pmd-plugin. See CPD.addFile(int, File) for the cause, i.e. the code near line 97 where it prints "Skipping

since it appears to be a symlink".