go/types: gotype to always report the same first error now
The old code may have reported different errors given an
erroneous package depending on the order in which files
were parsed concurrently. The new code always reports
errors in "file order", independent of processing order.
Also:
- simplified parsing code and internal concurrency control
- removed -seq flag which didn't really add useful functionality
Change-Id: I18e24e630f458f2bc107a7b83926ae761d63c334
Reviewed-on: https://go-review.googlesource.com/37655 Reviewed-by: Alan Donovan <adonovan@google.com>