gopls/internal/cache: express packageHandle as a state machine

In preparation for storing active packages on packageHandle, express the
various package handle states using a new packageState type, and hold on
to package handle data even if their local files may have changed, as
long as their metadata did not change.

Also: rename buildPackageHandle to evaluatePackageHandle, which better
matches its meaning, and move the package ID index to the View, since it
is shared across all snapshots.

Change-Id: I2c14613d320b1121f20bb3960d42370bef5ad98b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/614164
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>