WithStack not declared by package errors

### What happened?
I generated models & resolvers and implemented several resolver's methods.
In these methods I used `errors.WithStack(err)` from `github.com/pkg/errors` package.

When I run generate one more time, it automatically replaces `github.com/pkg/errors` with `errors` and after that complaining that method not declared.

### What did you expect?

I expect that it won't change import for `github.com/pkg/errors` package.

### Minimal graphql.schema and models to reproduce

Schema:
```graphql
type Query {
myUser(email: String!): User!
}

type User {
id: String!
email: String!
}
```

Resolvers with implementation
```go
package graph

// This file will be automatically regenerated based on the schema, any resolver implementations
// will be copied through when generating and any unknown code will be moved to the end.
// Code generated by github.com/99designs/gqlgen version v0.17.22

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

"github.com/bynov/test-graphql/graph/model"
)

// MyUser is the resolver for the myUser field.
func (r *queryResolver) MyUser(ctx context.Context, email string) (*model.User, error) {
if email == "1" {
return nil, errors.WithStack(errors.New("test"))
}

return &model.User{
ID: "1",
Email: email,
}, nil
}

// Query returns QueryResolver implementation.
func (r *Resolver) Query() QueryResolver { return &queryResolver{r} }

type queryResolver struct{ *Resolver }
```

gclgen.yml
```yml
# Where are all the schema files located? globs are supported eg src/**/*.graphqls
schema:
- graph/*.graphqls

# Where should the generated server code go?
exec:
filename: graph/generated.go
package: graph

# Uncomment to enable federation
# federation:
# filename: graph/federation.go
# package: graph

# Where should any generated models go?
model:
filename: graph/model/models_gen.go
package: model

# Where should the resolver implementations go?
resolver:
layout: follow-schema
dir: graph
package: graph

# Optional: turn on use ` + "`" + `gqlgen:"fieldName"` + "`" + ` tags in your models
# struct_tag: json

# Optional: turn on to use []Thing instead of []*Thing
# omit_slice_element_pointers: false

# Optional: turn off to make struct-type struct fields not use pointers
# e.g. type Thing struct { FieldA OtherThing } instead of { FieldA *OtherThing }
# struct_fields_always_pointers: true

# Optional: turn off to make resolvers return values instead of pointers for structs
# resolvers_always_return_pointers: true

# Optional: set to speed up generation time by not performing a final validation pass.
# skip_validation: true

# gqlgen will search for any type names in the schema in these go packages
# if they match it will use them, otherwise it will generate them.
autobind:
# - "github.com/bynov/test-graphql/graph/model"

# This section declares type mapping between the GraphQL and go type systems
#
# The first line in each type will be used as defaults for resolver arguments and
# modelgen, the others will be allowed when binding to fields. Configure them to
# your liking
models:
ID:
model:
- github.com/99designs/gqlgen/graphql.ID
- github.com/99designs/gqlgen/graphql.Int
- github.com/99designs/gqlgen/graphql.Int64
- github.com/99designs/gqlgen/graphql.Int32
Int:
model:
- github.com/99designs/gqlgen/graphql.Int
- github.com/99designs/gqlgen/graphql.Int64
- github.com/99designs/gqlgen/graphql.Int32
```

Adding new type:
```graphql
type NewUser {
id: String!
}
```

Run generate `go run github.com/99designs/gqlgen generate` and got error: `validation failed: packages.Load: /Users/bynov/go/src/github.com/bynov/test-graphql/graph/schema.resolvers.go:17:22: WithStack not declared by package errors`

### versions
- `go run github.com/99designs/gqlgen version v0.17.22`
- `go version go1.19.4 darwin/amd64`