internal/lsp/diagnostics: avoid erroneous error message for orphaned files
internal/lsp/diagnostics: avoid erroneous error message for orphaned files

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 <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools