[typescript] Allow mapping of external packages to local directories

**Is your feature request related to a problem? Please describe.**

We have some fragments in a shared package within a monorepo. Unless we explicitly symlink the directories, `graphql-codegen` can't find the fragments and will report `Unknown fragment xxxx`. Symlinking fixes this issue.

**Describe the solution you'd like**

I'd like to provide a list of replacement file paths for packages, e.g.
```yml
types.ts
importMapping:
@monorepo/fragments-package: '../fragments-package'
```

**Describe alternatives you've considered**

Symlinking works, but it's a long and arduous step for people setting up their repo, and we'd prefer not to require that.