cmd/go/internal/modload: use vendorMap in findModule
The build list is very incomplete in vendor mode,
so we can't rely on it in general.
findModule may be called in modload.PackageModuleInfo, which
load.LoadImport invokes relatively early during a build.
Before this change, the accompanying test failed at 'go build
-mod=vendor' with the message:
build diamondpoint: cannot find module for path diamondpoint