Limited Context tries to instantiate invalid plugins

When loading TIFF files using the standard SCIFIO Context (which does not include ImageJ services), SCIFIO needs the list of Codec objects, which triggers a call to ObjectIndex.get(Codec.class), which attempts to resolve all pending objects. More specifically, in our setting this includes all net.imagej.tool.Tool plugins, many of which rely on ImageJ-specific services not present in a SCIFIO-only Context.