fix(GraphQL): fix internal Aliases name generation

[minhaj-shakeel]

This PR modifies how the internal graph aliases are generated in case of multiple aliases of same field in the query.
For the given graphql query:

query {
      queryAuthor {
        name
        p1: posts(filter: {isPublished: true}){
          title
          text
        }
        p2: posts(filter: {isPublished: true}){
          title
          text
        }
      }
    }

earlier it was rewritten into:

query {
      queryAuthor(func: type(Author)) {
        name : Author.name
        posts : Author.posts @filter(eq(Post.isPublished, true)) {
          title : Post.title
          text : Post.text
          dgraph.uid : uid
        }
        posts1 : Author.posts @filter(eq(Post.isPublished, true)) {
          title : Post.title
          text : Post.text
          dgraph.uid : uid
        }
        dgraph.uid : uid
      }
    }

Now it is changed to:

query {
      queryAuthor(func: type(Author)) {
        name : Author.name
        posts : Author.posts @filter(eq(Post.isPublished, true)) {
          title : Post.title
          text : Post.text
          dgraph.uid : uid
        }
        posts.1 : Author.posts @filter(eq(Post.isPublished, true)) {
          title : Post.title
          text : Post.text
          dgraph.uid : uid
        }
        dgraph.uid : uid
      }
    }

Notice that new alias for second posts query is posts.1 instead of posts1. This is done to avoid issues in case if the Author have any field named posts1. Now posts.1 cannot be a field of Author as it results in a syntax error.

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