apollo client:codegen --namespace is not working for TypeScript

**Intended outcome:**
The generated type definition should be wrapped in the given namespace.

**Actual outcome:**
The generated file is not wrapped in the namespace.

**How to reproduce the issue:**

apollo client:codegen `--namespace=MyGQL` --addTypename --passthroughCustomScalars --customScalarsPrefix Scalar --outputFlat --target=typescript ./graphql/types.ts

All of the definition is not wrapped in `MyGQL` namespace.

**Versions**

apollo/2.8.3 linux-x64 node-v10.15.3

Start at the `apollo client:codegen` TypeScript code-generation entry point and reproduce the issue with the command shown, using `./graphql/types.ts` as the output. Done means the generated definitions are wrapped in the requested `MyGQL` namespace while preserving the other command options.

Written by the indexing model from the issue text.