cmd/go/internal/modload: clean up error reporting

• Consolidate 'if ld.AllowErrors' conditions into an 'ld.errorf'
  method.

• Rename SilenceErrors to SilencePackageErrors and clarify its
  documentation. (There is currently no way to silence errors in the
  module graph. Perhaps we should add one, but for now let's at least
  clarify the existing behavior.)

• Move 'tidy -v' verbose logging into LoadPackages, where other
  logging happens.

• Make checkMultiplePaths a loader method (since it only matters
  during package loading anyway).

• Check package and module-graph errors in loadFromRoots instead of
  LoadPackages. These checks were previously omitted on the
  ImportFromFiles path, which seems likely to be a bug. (We now
  suppress package errors explicitly in ImportFromFiles, which at
  least makes the bug more explicit.)

This somewhat simplifies the code structure in preparation for
the lazy-mode tidy implementation.

For #36460

Change-Id: I3ce3586c6934989d5194f00f99e7cc4423cf767f
Reviewed-on: https://go-review.googlesource.com/c/go/+/313229


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>