Prevent duplicate path names in a project's PYTHONPATH when creating src...

... folders.

Previously, if a PyDev Project source folder was created with the same name as a path already
present in the project's PYTHONPATH, the created folder's path would be added as a duplicate.
Prevent this from happening by checking for existing pathnames before adding a new one.

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I had made this change in a different patch, but since it didn't really belong there I'm putting it here on its own.