Support using a module's external name to run IPM commands

[isc-eneil]

Several HS use cases require a module to have an external name that is customer-facing which differs from the module's internal name which is developer-facing. Customers should be able to run IPM command using a module's external name.

After discussion between HSDevOps and AppServices, we came up the following <ExternalName> constraints:

• A package's externalname is unique within the namespace
• A package's externalname can't conflict with another package's real name

cc @isc-kiyer @isc-jili @isc-jlechtne @isc-shuliu @timleavitt