add support for custom scalars -- mainly adding proper import machinery

generate/operation.go.tmpl

@@ -2,16 +2,7 @@ package {{.Config.Package}}
 
 // Code generated by github.com/Khan/genqlient, DO NOT EDIT.
 
-import (
-    {{if .Config.ContextType -}}
-    "{{.Config.ContextPackage}}"
-    {{end}}
-    {{- if .ImportJSON -}}
-    "encoding/json"
-    {{end}}
-
-    "github.com/Khan/genqlient/graphql"
-)
+{{.Imports}}
 
 {{/* TODO: type-assert that your ctx type implements context.Context */}}
 
@@ -21,10 +12,10 @@ import (
 {{.Doc}}
 func {{.Name}}(
     {{if $.Config.ContextType -}}
-    ctx {{$.Config.ContextTypeReference}},
+    ctx {{ref $.Config.ContextType}},
     {{end}}
     {{- if not $.Config.ClientGetter -}}
-    client graphql.Client,
+    client {{ref "github.com/Khan/genqlient/graphql.Client"}},
     {{end}}
     {{- range .Args -}}
     {{.GoName}} {{.GoType}},