[gql_build] Support shared fragments

[MattGson]

As far as I can tell, there is no way to have multiple queries use shared fragments
i.e.

fragment someFragment on C {
fields
}

query A {
someField
...someFragment
}

query B {
someField
...someFragment
}

A normal use of fragments is to extract shared pieces of queries.
This is pretty important in dart as types are not structural like other languages such as Typescript.
That means if I want to render a shared widget using the query data, I cannot mix and match data from query A and query B even if the types are structurally the same.

[MattGson]

Was able to do this using #import "./MyFragment.graphql" at the top of each query file.