gopls/internal/hooks: add Staticcheck's quickfix category of checks

Staticcheck's new quickfix category contains optional code
refactorings, such as turning a chain of if/else-if into a switch
statement, or simplifying boolean expressions.

These checks produce diagnostics with the "hint" or "information"
severities. Most use "hint", which produces a subtle hint in VSCode's
UI that a refactoring is available. "information" produces a blue
underline and is only used for those checks that have a very high
likelyhood of improving code readability.

Change-Id: I8f5b4eca67eb6b4e45c17510459d83eb673384e3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/322492
Trust: Dominik Honnef <[email protected]>
Run-TryBot: Dominik Honnef <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

internal/lsp/cache: trim more stuff in ParseExported mode

Despite the name, ParseExported only hollowed out declarations -- it
didn't actually drop any from the AST. This leaves a fair amount of
unexported crud behind. Unfortunately, there are a *lot* of ways to
expose an unexported declaration from an exported one, and it can be
done across files. Because of that, discarding unexported declarations
requires a lot of work.

This CL implements a decent attempt at pruning as much as possible from
the AST in ParseExported mode.

First, we analyze the AST of all the files in the package for exported
uses of unexported identifiers, iterating to a fixed point. Then, we
type check those ASTs. If there are missing identifiers (probably due to
a bug in the dependency analysis) we use those errors to re-parse. After
that we give up and fall back to the older, less effective trimming. The
pkg type changes slightly to accomodate the new control flow.

We have to analyze all the files at once because an unexported type
might be exposed in another file. Unfortunately, that means we can't
parse a single file at a time any more -- the result of parsing a file
depends on the result of parsing its siblings. To avoid cache
corruption, we have to do the parsing directly in type checking,
uncached.

This, in turn, required changes to the PosTo* functions. Previously,
they operated just on files, but a file name is no longer sufficient to
get a ParseExported AST. Change them to work on Packages instead. I
squeezed in a bit of refactoring while I was touching them.

Change-Id: I61249144ffa43ad645ed38d79e873e3998b0f38d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/312471
Trust: Heschi Kreinick <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>