Support directory move

[jayvdb]

If I move a directory and run wstool scrape, it will add a second entry to .rosinstall.

It would be nice if there was a wstool mv from to, or wstool scrape checked for an existing entry with the same remote name, emitted a warning and did a move if the user confirms that is what was intended.

[tkruse]

Should be sufficient if scrape detected configured repositories that are missing from filesystem and offered to remove those I guess. That should be easier to implement and have the same effect, covering other usecases as well.

[dirk-thomas]

Closing since the repository is about to be archived: see #154.