Fix issue where skipIfSpecIsUnchanged was ignored when using a classpath input spec

Created by: dhutchison

The skipIfSpecIsUnchanged configuration setting to the maven plugin did not work when the input spec came from a classpath resource, which could lead to infinite build loops when the plugin was used in eclipse.

Fix #5805 (closed)

The CodeGenMojo appears to have already had support for creating hashes of input specs from different locations, but an incorrect if check was preventing these non-filesystem locations having hashes created.

Two additional test methods have been added to test that a spec from the classpath can respect the skipIfSpecIsUnchanged setting.

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