x/tools/gopls: vet the DegradeClosed memory mode and move out of experimental

[findleyr]

https://golang.org/cl/330529 fixed loading of std and cmd, resulting in better features for std and cmd but worse resource usage.

"memoryMode": "DegradedClosed" should per my recollection revert us to the old model, but it seems not to work. We should fix this. More generally, I think the degraded memory model could be useful for many users, and we should vet it and move it out of experimental.

CC @stamblerre

[gopherbot]

Change https://golang.org/cl/333689 mentions this issue: internal/lsp: improve package search in a couple places

[gopherbot]

Change https://golang.org/cl/336410 mentions this issue: internal/lsp: in degraded mode, limit the workspace to active packages