If the user changes the extension of a go file while editing,
the existing code complains 'No packages found for open file'.
This error messsag is not helpful and this CL avoids it.
Change-Id: Idf426f1ce8b83b23757f81aa1f322fea86df9d82
Reviewed-on: https://go-review.googlesource.com/c/tools/+/350789
Run-TryBot: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>