Read service gqlgen.yml file to find a list of related graphql schemas

Summary:
In order to satisfy `make gqlgen`, we needed to add a shared '@Migrate'
directive that services can reference (see D62237). We then ran into
errors linting this shared file because it duplicates the `migrate`
directive we manually add as part of the linter here.

To fix, I resolved the TODO here to just read `gqlgen.yml` for the list
of schema files that each service depends on.

Test Plan:
With these changes applied in `khan-linter` and with webapp including
the changes from `D62237` `ka-lint
pkg/graphql/shared-schemas/side_by_side_directives.graphql` and saw no
errors.

I then ran `make check` within `services/content` and likewise saw no
error.

Reviewers: csilvers, marksandstrom

Reviewed By: csilvers

Subscribers: rachelmarincola, kevinb

Differential Revision: https://phabricator.khanacademy.org/D62244

linters.py

@@ -2,7 +2,6 @@
 """Linters process files or lists of files for correctness."""
 
 import itertools
-import glob
 import json
 import logging
 import os
@@ -840,11 +839,10 @@ def process(self, f, contents_of_f):
         #    https://github.com/cjoudrey/graphql-schema-linter/issues/210
         #
         # So instead we take a two-pronged approach:
-        # 1) We append all the other schema files in our directory.
-        #    This matches what gqlgen does, and will not have the
-        #    @extends problem since it's only for a single backend.
-        #    TODO(csilvers): read gqlgen.yml to figure out what
-        #    schema files to include, instead of glob.
+        # 1) We find the `gqlgen.yml` file that belongs to this service and
+        #   parse it to find out all the other schema files the service depends
+        #   on. This matches what gqlgen does, and will not have the @extends
+        #   problem since it's only for a single backend.
         # 2) We then try to run the linter and catch all "undefined
         #    type" errors.  We then add fake definitions for those
         #    types, and re-run the linter.
@@ -861,12 +859,22 @@ def process(self, f, contents_of_f):
         directive @provides(fields: _FieldSet!) on FIELD_DEFINITION
         directive @key(fields: _FieldSet!) on OBJECT | INTERFACE
         directive @extends on OBJECT | INTERFACE
-
-        # Khan-specific directives
-        directive @migrate(from: String!, state: String!) on FIELD_DEFINITION
         """
+        # All schema files should live at the top level of a service, so we can
+        # look for the current gqlgen.yml
+        service_root = os.path.dirname(f)
+        gqlgen_path = os.path.join(service_root, "gqlgen.yml")
+
+        try:
+            gqlgen_contents = yaml.safe_load(self._read_file(gqlgen_path))
+            schema_files = [
+                os.path.join(service_root, schema_file)
+                for schema_file in gqlgen_contents["schema"]]
+        except Exception:
+            # If the gqlgen.yml file doesn't exist (for shared schemas) we
+            # assume there are no related files we need.
+            schema_files = []
 
-        schema_files = glob.glob(os.path.join(os.path.dirname(f), '*.graphql'))
         for other_f in schema_files:
             if other_f == f:
                 continue