This CL improves the usability of the "Toggle compiler optimization
details" code action by:

- making it work for _test.go files too (by running "go test -c"
  instead of "go build");
- changing the flag to be per directory, not per metadata.Package,
  as this led to confusing behaviour w.r.t. in-package test files.
- presenting specific "Show"/"Hide" command titles depending on
  the current state, instead of the vague "Toggle".

It includes a test for all the new functionality.

Also, document the "transitively error free" requirement.

Fixes golang/go#71106
Fixes golang/go#42812 (amusingly)

Change-Id: I842d7e3f53d1c4f0e94b8741ca792c3d999d6ff3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/640395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>