Refactor import-loading to simplify the type-generation code (#101)

## Summary:
genqlient has some code (`imports.go`) dedicated to tracking which
imports we need and avoiding conflicts, as well as converting a
(restricted) Go expression like `map[string]github.com/me/mypkg.MyType`
to an import (`github.com/me/mypkg`) and a type-reference
(`map[string]mypkg.MyType`) to be used in the context of that import,
and at least making some attempt to track conflicts.  (Right now the
conflict-avoidance is not very smart, and not very well tested, but it
comes up rarely anyway.)  Sadly, that code was a bit cumbersome to use,
because you had to first register the imports (typically from
`convert.go`), then use them (often from the template).

In this commit I refactor the order we write things in order to allow a
significant simplification of how we import; in particular we no longer
have to guess in advance what imports which template will need; it can
just do `{{ref <expr>}}` as before, and it just works.  To do this, I:
- changed the importer to have only one API, which adds the import if
  needed, and returns the reference either way
- added a check that we don't add imports after they're written
- reorganized the toplevel templates a bit to make sure that check never
  fires; we now generate all the types and operations, then write the
  imports and glue it all together
This removes a bunch of silly code, and should simplify the process of
adding custom (un)marshalers (#38).

While I was at it, I put the documentation of what expressions we
support in a more visible place, and added a type-assertion that your
custom context type implements context.Context (if applicable).

## Test plan:
make check


Author: benjaminjkraft

Reviewers: StevenACoffman, dnerdy, aberkan, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: StevenACoffman, dnerdy

Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/101

generate/operation.go.tmpl

@@ -1,20 +1,9 @@
-package {{.Config.Package}}
-
-// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
-
-{{.Imports}}
-
-{{/* TODO: type-assert that your ctx type implements context.Context */}}
-
-{{.Types}}
-
-{{range .Operations}}
 {{.Doc}}
 func {{.Name}}(
-    {{if ne $.Config.ContextType "-" -}}
-    ctx {{ref $.Config.ContextType}},
+    {{if ne .Config.ContextType "-" -}}
+    ctx {{ref .Config.ContextType}},
     {{end}}
-    {{- if not $.Config.ClientGetter -}}
+    {{- if not .Config.ClientGetter -}}
     client {{ref "github.com/Khan/genqlient/graphql.Client"}},
     {{end}}
     {{- range .Args -}}
@@ -46,8 +35,8 @@ func {{.Name}}(
     {{end -}}
 
     var err error
-    {{if $.Config.ClientGetter -}}
-    client, err := {{ref $.Config.ClientGetter}}({{if ne $.Config.ContextType "-"}}ctx{{else}}{{end}})
+    {{if .Config.ClientGetter -}}
+    client, err := {{ref .Config.ClientGetter}}({{if ne .Config.ContextType "-"}}ctx{{else}}{{end}})
     if err != nil {
         return nil, err
     }
@@ -55,7 +44,7 @@ func {{.Name}}(
 
 	var retval {{.ResponseName}}
     err = client.MakeRequest(
-        {{if ne $.Config.ContextType "-"}}ctx{{else}}nil{{end}},
+        {{if ne .Config.ContextType "-"}}ctx{{else}}nil{{end}},
         "{{.Name}}",
         `{{.Body}}`,
         &retval,
@@ -63,4 +52,3 @@ func {{.Name}}(
     )
     return &retval, err
 }
-{{end}}