This commit allows installing a package when a different version is
already installed. Currently, if the package is installed globally (the
only option) - it will overwrite the previous version reference.
A followup change will allow installing into environments to work around
this limitation.

Part of #660

Also:

* Add PackageManager.IsInstalled - fuzzy-match installation check for packages, without parsing their manifests

Signed-off-by: Oren Shomron <shomron@gmail.com>