Put full ImportDecls in ModSummary instead of just ModuleNames

Put full ImportDecls in ModSummary instead of just ModuleNames
... and use it to make ghc -M generate correct cross-package
dependencies when using package-qualified imports (needed for the new
build system).  Since we're already parsing the ImportDecl from the
source file, there seems no good reason not to keep it in the
ModSummary, it might be useful for other things too.