Reload config packages after generating models

If models are generated in a package that has already been loaded, and
that package refers to another package that has already been loaded, we
can find ourselves in a position where it appears that a GQL `union` is
not satisfied.

For example, if we have:

```
union Subject = User
```

with this gqlgen.yml in github.com/wendorf/gqlgen-error/gql:

```
schema:
- schema.graphql
exec:
  filename: generated.go
model:

  filename: models_gen.go
models:
  User:
    model: github.com/wendorf/gqlgen-error/gql.User
  Subject:
    model: github.com/wendorf/gqlgen-error/models.Subject
```

Note that our User model is in the github.com/wendorf/gqlgen-error.gql
package, and our models_gen.go will be generated in that same package.

When we try to run gqlgen, we get this error:

```
merging type systems failed: unable to bind to interface: github.com/wendorf/gqlgen-error/gql.User does not satisfy the interface github.com/wendorf/gqlgen-error/models.Subject
```

Digging deeper, it's because we use types.Implements in
codegen/interface.go, which does a shallow object comparison. Because
the type has been reloaded, it refers to a _different_ interface type
object than the one we're comparing against, and get a false negative.

By clearing the package cache and repopulating it, the whole package
cache is generated at the same time, and comparisons across packages
work.

To see a demo of this, check out
https://github.com/wendorf/gqlgen-error and try the following:

1. Checkout the works-with-v0.10.2 branch and `go generate ./...` to see
   that it works
2. Checkout the breaks-with-v0.13.0 branch (or run go get
   github.com/99designs/gqlgen@v0.13.0 yourself) and `go generate ./...`
   to see errors
3. Checkout the works-with-pull-request branch and `go generate ./...`
   to see that it works again. This branch adds a go.mod replace
   directive to use the gqlgen code in this PR.

The demo starts at v0.10.2 since it is the last release without this
problem. #1020 introduces the
code that fails in this scenario.

codegen/config/config.go

@@ -194,15 +194,8 @@ func (c *Config) Init() error {
 	}
 
 	c.injectBuiltins()
-
 	// prefetch all packages in one big packages.Load call
-	pkgs := []string{
-		"github.com/99designs/gqlgen/graphql",
-		"github.com/99designs/gqlgen/graphql/introspection",
-	}
-	pkgs = append(pkgs, c.Models.ReferencedPackages()...)
-	pkgs = append(pkgs, c.AutoBind...)
-	c.Packages.LoadAll(pkgs...)
+	c.Packages.LoadAll(c.packageList()...)
 
 	//  check everything is valid on the way out
 	err = c.check()
@@ -213,6 +206,20 @@ func (c *Config) Init() error {
 	return nil
 }
 
+func (c *Config) packageList() []string {
+	pkgs := []string{
+		"github.com/99designs/gqlgen/graphql",
+		"github.com/99designs/gqlgen/graphql/introspection",
+	}
+	pkgs = append(pkgs, c.Models.ReferencedPackages()...)
+	pkgs = append(pkgs, c.AutoBind...)
+	return pkgs
+}
+
+func (c *Config) ReloadAllPackages() {
+	c.Packages.ReloadAll(c.packageList()...)
+}
+
 func (c *Config) injectTypesFromSchema() error {
 	c.Directives["goModel"] = DirectiveConfig{
 		SkipRuntime: true,

internal/code/packages.go

@@ -26,6 +26,13 @@ type Packages struct {
 	numNameCalls int // stupid test steam. ignore.
 }
 
+// ReloadAll will call LoadAll after clearing the package cache, so we can reload
+// packages in the case that the packages have changed
+func (p *Packages) ReloadAll(importPaths ...string) []*packages.Package {
+	p.packages = nil
+	return p.LoadAll(importPaths...)
+}
+
 // LoadAll will call packages.Load and return the package data for the given packages,
 // but if the package already have been loaded it will return cached values instead.
 func (p *Packages) LoadAll(importPaths ...string) []*packages.Package {

plugin/modelgen/models.go

@@ -214,13 +214,22 @@ func (m *Plugin) MutateConfig(cfg *config.Config) error {
 		b = m.MutateHook(b)
 	}
 
-	return templates.Render(templates.Options{
+	err := templates.Render(templates.Options{
 		PackageName:     cfg.Model.Package,
 		Filename:        cfg.Model.Filename,
 		Data:            b,
 		GeneratedHeader: true,
 		Packages:        cfg.Packages,
 	})
+	if err != nil {
+		return err
+	}
+
+	// We may have generated code in a package we already loaded, so we reload all packages
+	// to allow packages to be compared correctly
+	cfg.ReloadAllPackages()
+
+	return nil
 }
 
 func isStruct(t types.Type) bool {