separateOperations: stop unused fragments from being included

[filipstefansson]

The separateOperations method sometimes include unused fragments if you name an operation the same thing as a fragment:

fragment One on T {
  oneField
}

fragment Two on T {
  twoField
}

query One {
  ...Two
}

query Ones {
  ...One
}

When running separateOperations on the above, and then looking at .Ones of the result, we get:

fragment One on T {
  oneField
}

## NOT SUPPOSED TO BE HERE ##
fragment Two on T {
  twoField
}

query Ones {
  ...One
}

This happens because in the dependency graph, there's nothing differentiating an operation from a fragment. They are just added by name. So in the scenario above the fragment One will add the query One as a dependecy of Ones, and then the query One has the fragment Two as a dependency, so that will be added too.

I fixed this by adding a prefix to the names in the dependecy graph, so operations can't be added a dependencies anymore, only fragments.

[IvanGoncharov]

@filipstefansson Thanks for taking the time to report it and provide a solution.
Adding prefixes is a valid approach but I decided to take this opportunity and refactor this function in #2859.