addendum: overrides apply if value matches, as well as key

Found a pretty serious problem with overrides while starting
implementation.  Namely, it is in many cases impossible to determine
whether a node in a reified tree was subject to an override or not.

The edge case is described in this patch, and a fix proposed:

> **If a dependency matches the `key` in an overrides object _or_ it
> matches the `"."` value specifier, then the override ruleset will
> apply.**

By doing this, it prevents version-swapping, makes infinite regress even
more impossible, and ensures that the resulting package tree can always
be properly examined, without any out of band bookkeeping.

cc: @nlf

PR-URL: #441
Credit: @isaacs
Close: #441
Reviewed-by: @isaacs

Author: isaacs