In `cmDepends` use
`typedef std::map<std::string, std::vector<std::string>> DependencyMap`
instead of defining a
`class DependencyVector : public std::vector<std::string>`
and using it in `std::map<std::string, DependencyVector>`.

Since `std::map<std::string, std::vector<std::string>>` is used in various
other places, we now reuse all of it's auto generated methods.  This doesn't
happen when we use `DependencyVector` in a `std::map`, because it is a
different class than `std::vector<std::string>`.