Clean up a provided absolute or relative directory path.

This prettification relies on basic operations such as coalescing multiple forward slashes into a single slash, removing '.' and './' current directory segments, and removing parent directory whenever '..' is encountered. If not empty, the returned path ends with a forward slash.

For instance, this will turn "d1/s1///s2/..//../s3" into "d1/s3/".

This only performs a string based analysis of the path. No checks are done to make sure the path actually makes sense from the file system perspective.