Generate imports wrong package and gives error

### What happened?
When running `go run github.com/99designs/gqlgen generate` the generated resolver functions imports the wrong package.

if I use the [github.com/pkg/errors](https://github.com/pkg/errors) library in my resolvers and then later on I trigger the `generate` command, gqlgen will remove this import and instead import the standard library's `errors` package.

**PRE `go run github.com/99designs/gqlgen generate`**
```go
// user.resolvers.go

import (
"context"
"github.com/pkg/errors"

"github.com/foo/bar/app"
)

func (r *queryResolver) Me(ctx context.Context) (*app.User, error) {
user,err := app.UserFromContext(ctx)
if err != nil {
return nil, errors.Wrap(err, "no user in context")
}
return user, nil
}
```

**POST `go run github.com/99designs/gqlgen generate`**

```sh
validation failed: packages.Load: /github.com/foo/bar/graph/resolver/user.resolvers.go: Wrap not declared by package errors
```

Updated generated file...
```go
// user.resolvers.go

import (
"context"
"errors"

"github.com/foo/bar/app"
)

func (r *queryResolver) Me(ctx context.Context) (*app.User, error) {
user,err := app.UserFromContext(ctx)
if err != nil {
return nil, errors.Wrap(err, "no user in context")
}
return user, nil
}
```

So even though stdlib `errors` pkg doesn't have a `Wrap` function, it will still be added to the import statement rather than using the one that was previously declared `github.com/pkg/errors` followed by reporting that the function is not declared in that package.

**gqlconfig.yml**
```yml
schema:
- "graph/schema/**/*.graphql"

exec:
filename: graph/generated/generated.go
package: generated

resolver:
layout: follow-schema
dir: graph/resolver
package: resolver

struct_tag: gql
omit_slice_element_pointers: true

autobind:
- github.com/foo/bar/app
```

### What did you expect?
import / use the already declared package rather than attempting to import it's own

### Minimal graphql.schema and models to reproduce
```graphql
# schema/schema.graphql
schema {
query: Query
mutation: Mutation
}

type Query

type Mutation

```

```graphql
# schema/types/user.graphql
extend type Query {
me: User
}
type User {
id: ID!
email: String!
name: String
}
```

### versions
- `gqlgen version`? v0.11.3
- `go version`? 1.14
- dep or go modules? go mod

问题出在处理导入的代码生成逻辑中。查看 resolver 生成代码，它很可能位于 `generate` 包中。测试用例涉及一个使用 `github.com/pkg/errors` 的 resolver。在最小示例上运行生成过程以复现问题，然后跟踪导入的解析和重写过程。修复应在使用 `errors.Wrap` 时保留正确的导入。

由索引模型根据 Issue 内容生成。