fix: avoid double loading of source package

[3vilhamster]

Hey.
Thank you for the super helpful package.
We are heavily using it in github.com/cadence-workflow/cadence for wrapper generations. One thing though, in our case it is a bit slow.
I was investigating possibilities - and found that sourcePackage is read 2 times:

1. inside getOptions() function
2. inside NewGenerator consructor

Removing this double loading saves around 10% of the execution time.

This should be a noop for other cases.

[hexdigest]

Hi @3vilhamster! This is a good one, thanks!