Fix DEFSYSTEM printed package in system files for single-file projects.

ASDF 3 exports a symbol named "DEFSYSTEM" from ASDF, but its home
package is named ASDF/DEFSYSTEM. When naively printed into a generated
system file, it looks like this:

    (asdf/defsystem:defsystem ...)

This is incompatible with older versions of ASDF.

This commit overrides printing of external ASDF symbols so they always
appear as "ASDF:FOO" rather than whatever their real home package is.

Thanks to Stelian Ionescu for the pprint-dispatch-table approach.