Currently, `COMPLETE` pragmas are not read from external packages at
all, which quite limits their usefulness. This extends
`ExternalPackageState` to include `COMPLETE` sets from other packages,
and plumbs around the appropriate values to make it work the way you'd
expect it to.

Fixes #13350.

Test Plan: make test TEST=T13350

Reviewers: rwbarton, mpickering, austin, simonpj, bgamari

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D3257