fix loading tests in Open Source version

This required giving up on the len(patterns) = len(loaded) assumption,
which in turn required refactoring how the concurrency is structured.

I tested that edge cases like -parallel_jobs=1 and -parallel_jobs=35
(for a conversion with 35 packages) work correctly.

Also consider .pb.go files as generated while here.

fixes golang/open2opaque#3
fixes golang/open2opaque#4

PiperOrigin-RevId: 707109446
Change-Id: I2241e4a9a54613e68efee437b691946c66a3bfd3
Reviewed-on: https://go-review.googlesource.com/c/open2opaque/+/637135
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>