We used to use build.Import to get the dependencies, but that meant
we had to repeat the check for every possible GOOS/GOARCH/cgo
combination, which took too long. So we made the test in short mode
only check the current GOOS/GOARCH/cgo combination.
But some combinations can't run the test at all. For example darwin/arm64
does not run tests with a full source file systems, so it cannot test itself,
so nothing was testing darwin/arm64. This led to bugs like #10455
not being caught.

Rewrite the test to read the imports out of the source files ourselves,
so that we can look at all source files in a directory in one pass,
regardless of which GOOS/GOARCH/cgo/etc they require.
This one complete pass runs in the same amount of time as the
old single combination check ran, so we can now test all systems,
even in short mode.

Change-Id: Ie216303c2515bbf1b6fb717d530a0636e271cb6d
Reviewed-on: https://go-review.googlesource.com/12576Reviewed-by: Ian Lance Taylor <iant@golang.org>