Omit types not used by loaded resolvers

[MichalLytek]

This PR fix the leakages in generated schema that were unintended. Now, the buildSchema function will produce GraphQLSchema only with operations declared in classes provided in resolvers option 🎉

This also apply to other types (object, inputs, interfaces) - they will be emitted in schema only if they are explicitly used in used types.
So if you have declared 10 object types implementing an interface but don't use them directly as a return type of an operation, part of an union type or as a field type, they will be omitted too.

In that case you might want to provide the orphaned types explicitly to buildSchema option to emit them too as they might be used indirectly, e.g. by the underlying database or something:

import { FirstResolver, SecondResolver } from "../app/src/resolvers";
import { FirstObject } from "../app/src/types";
// ...
const schema = await buildSchema({
  resolvers: [FirstResolver, SecondResolver],
  // here provide all the types that are missing in schema
  orphanedTypes: [FirstObject],
});

And because we don't have control of glob-imported files with resolvers and types, this feature doesn't apply if you use the indirect, glob path version of resolvers options property. In that case it will still emit the whole metadata storage content as earlier 😞

Closes #110 🔒

[codecov]

Codecov Report

Merging #415 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   94.79%   94.66%   -0.13%     
==========================================
  Files          71       71              
  Lines        1152     1163      +11     
  Branches      214      219       +5     
==========================================
+ Hits         1092     1101       +9     
- Misses         57       59       +2     
  Partials        3        3

Impacted Files	Coverage Δ
src/utils/buildSchema.ts	100% <100%> (ø)	⬆️
src/schema/schema-generator.ts	96.68% <100%> (-0.75%)	⬇️

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