internal/lsp/source: do not allow extraction of an import spec
internal/lsp/source: do not allow extraction of an import spec

Fixes golang/go#40635

Change-Id: Iab8ca37d251a95334c19f32873f9ba9cc1bdd2f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/251018
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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