We still hear from users for whom gopls uses too much memory. My efforts
to reduce memory usage while maintaining functionality are proving
fruitless, so perhaps it's time to accept some functionality loss.

DegradeClosed MemoryMode typechecks all packages in ParseExported mode
unless they have a file open. This should dramatically reduce memory
usage in monorepo-style scenarious, where a ton of packages are in the
workspace and the user might plausibly want to edit any of them.
(Otherwise they should consider using directory filters.)

The cost is that features that work across multiple packages...won't.
Find references, for example, will only find uses in open packages or in
the exported declarations of closed packages.

The current implementation is a bit leaky; we keep the ParseFull
packages in memory even once all their files are closed. This is related
to a general failure on our part to drop unused packages from the
snapshot, so I'm not going to try to fix it here.

Updates golang/go#45457, golang/go#45363.

Change-Id: I38b2aeeff81a1118024aed16a3b75e18f17893e2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/310170
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>