Before this change, many kinds of error would cause the loader to stop.
making it brittle when analyzing large codebases, as in "godoc -analysis".
This change moves operations that used to occur during
configuration---(*build.Context).Import, loading, and parsing of
initial packages---into the Load call, and ensures that all failures
during Loading are reported at the end so that the maximum amount of
progress is made.
Also: redesign the tests and add many new cases.
Change-Id: Ia8cd99416af7c5d4a5fe133908adfa83676d401f
Reviewed-on: https://go-review.googlesource.com/3626
Reviewed-by: Robert Griesemer <gri@golang.org>
0dda50d42a