internal/imports: pass dummy source for completion functions
internal/imports: pass dummy source for completion functions

The goimports code tries to read source from disk if none is supplied,
but the completion functions don't need source. Pass a dummy slice.

Fixes golang/go#36671.

Change-Id: Ieecc9ee75d39f8f165c7b9764ff49c8334a7fcd2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/215681
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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