internal/lsp/source: propose loaded unimported package names first
internal/lsp/source: propose loaded unimported package names first

When proposing unimported package members, we start with packages loaded
in-memory first, for three reasons: we know that they're fast, they're
fully typed, and they're probably pretty relevant. I hadn't bothered
doing that for package names, because the first two reasons aren't very
relevant. But the third still is -- loaded packages are a pretty good
approximation for in module scope, etc.

With this change we do package names the same as members, so relevance
should be about as good. Not perfect, but nobody's complained much yet.

Fair bit of copy-and-paste, but I don't want to extend the
getAllCandidates abstraction outside of the imports package yet.

Updates #38104.

Change-Id: Ia479181607dff898baee3cd6aa84d1ab61715d19
Reviewed-on: https://go-review.googlesource.com/c/tools/+/226559
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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