4af4487 added a mirror_id function to most FetchStrategy
implementations that is used to calculate resource locations in
mirrors. It left out BundleFetchStrategy which broke all packages
making use of BundlePackage (e.g. xsdk). This adds a noop
implementation of mirror_id to BundleFetchStrategy so that the
download/installation of BundlePackages can proceed as normal.
6ec39b6c81