ExternalData: Add support for custom algorithm-to-URL mapping

Allow URL templates to contain a %(algo:<key>) placeholder that is
replaced by mapping the canonical hash algorithm name through a map
defined by the <key>.

Extend the Module.ExternalData test to cover the behavior.
Extend the RunCMake.ExternalData test to cover error cases.

Change-Id: Iabbcb81de0253b4482bce65a0e32cc5c1e8a4816