Switch repo to a metapackage

[LeoColomb]

Ref:

• Drop installer requirement wordpress-packager#507
• RFC: roots/wordpress package switching the "no-content" WordPress release #32

---

• Closes RFC: roots/wordpress package switching the "no-content" WordPress release #32
• Closes Old themes not in WordPress core 5.1 are installed with roots/wordpress #3
• Closes Beta & Release Candidates Tags #20
• Closes Maybe add extensions/libraries required by Wordpress to composer file #12
• Closes Move updater code to a different repo #10

[LeoColomb]

@retlehs All the mentioned issues can be closed I guess, a little spring cleaning 😊

[atimmer]

Entirely unsure what's the right place to put this, so just noting it here:

This change breaks composer patches. It requires one to change from:

"patches": {
      "roots/wordpress": {
      }
}

to:

"patches": {
      "roots/wordpress-no-content": {
      }
}