[Go SDK] starcgen improvements

[damccorm]

The starcgen code generator works OK, but could do with some improvements.

• Uniquifying imports (handling multiple imports with same short suffix)
• Generating multiple iterNatives (eg when the normal symbol is already taken).
• Keying off of beam.Register* calls rather than command line.
  **  Avoids duplicating lists of identifiers, and improves default behavior.
  ** Possibly have a new beam.RegisterDoFn which can take a list of DoFn and struct types a function or a struct, and key off those, reducing boiler plate somewhat.
• Perhaps having a specific single import alias package for components required for import, rather than the current 3-4.
• Generate efficient Beam Schema coders for registered types?
• Handle SplittableDoFns properly.

Imported from Jira BEAM-9616. Original Jira may contain additional context.
Reported by: lostluck.