When working in a monorepo environment, it can be desirable to generate
Go SDKs into a structure less like the upstream SDKs, and more like
this:
github.com/x/mymonorepo/sdk/go/package-name
Where `package-name` is also the root of a Go module. Since
`package-name` is not a valid package name in Go, it's also desirable to
be able to choose a replacement name and reduce the amount of nesting.
This commit adds a new Go option to the schema, `rootPackageName`, which
can be used to modify the generated root package name (e.g. to
`mypackage` instead of `package-name`, and remove the additional layer
of nesting.
Test coverage is added to ensure that the correct file structure and
package names are generated.
