Code review for pg_dump's handling of ALTER INDEX ATTACH PARTITION.

Code review for pg_dump's handling of ALTER INDEX ATTACH PARTITION.

Ensure the TOC entry is marked with the correct schema, so that its
name is as unique as the index's is.

Fix the dependencies: we want dependencies from this TOC entry to the
two indexes it depends on, and we don't care (at least not for this
purpose) what order the indexes are created in.  Also, add dependencies
on the indexes' underlying tables.  Those might seem pointless given
the index dependencies, but they are helpful to cue parallel restore
to avoid running the ATTACH PARTITION in parallel with other DDL on
the same tables.

Discussion: https://postgr.es/m/10817.1535494963@sss.pgh.pa.us