getRelativePath fails on non-normalized inputs

The algo in AbstractSiteMojo.getRelativePath() assumes that its input paths are both normalized and produces wrong output if it gets something like "dir/../dir". Attached is a simple unit test and a fix for the method itself.

FYI, non-normalized path are easily created by Maven if one has a non-Maven-like directory layout with
project/
project-parent/
project-module-1/
where simple string/path concatenation produces a path like "project/project-module-1/../project-parent" when referencing the parent POM from the module POM.

Path/URL transformations like normalization/relativization/resolution are quite ubiquitous in Maven. Isn't there a nice util class around that prevents each and every plugin developer to reimplement those error-prone methods?