Previously, we used relative paths in submodules. When cloning from
GitHub, they had to be manually tweaked.
Since a76b233d we use absolute paths, so this workaround can be removed.

The code, including the generated module with the version, is now in
ghc-boot. Config.hs reexports stuff as needed, ghc-pkg doesn't need any
tricks at all.

This moves all URL references to Trac Wiki to their corresponding
GitLab counterparts.

This substitution is classified as follows:

1. Automated substitution using sed with Ben's mapping rule [1]
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...

2. Manual substitution for URLs containing `#` index
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz

3. Manual substitution for strings starting with `Commentary`
    Old: Commentary/XxxYyy...
    New: commentary/xxx-yyy...

See also !539

[1]: https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mapping.json

- Remove dph from validate; dph was removed
- The required-tag argument to boot was used only for dph, remove
- check_boot_packages() was not called at all, and didn't work.
  I fixed it based on previous Perl version.

Test Plan: Harbormaster

Reviewers: bgamari, thomie

Reviewed By: bgamari

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5129

Test Plan: Run `./boot`, check to make sure that
`libraries/ghc-prim/GNUmakefile` is sane

Subscribers: thomie, carter, sjakobi

Differential Revision: https://phabricator.haskell.org/D4580

D3918 neglected to implement this when it rewrote boot in python.