Do not propose Owners or Packages as Subscribers when uninstalled
Summary:
After creating Owners and Packages and then uninstalling the Owners and Packages applications, PhabricatorMetaMTAMailableFunctionDatasource proposals (for example when using typeahead in the "Subscribers" field on http://phorge.localhost/maniphest/query/advanced/) still propose results for these applications, plus the field placeholder text still mentions packages. I consider this unwanted and confusing behavior.
Closes T16149
Test Plan:
1. Go to http://phorge.localhost/applications/view/PhabricatorPackagesApplication/ and make sure that "Packages" prototype application is installed
2. Go to http://phorge.localhost/applications/view/PhabricatorOwnersApplication/ and make sure that "Owners" application is installed
3. Go to http://phorge.localhost/people/new/standard/ and create a Standard User called "TestPackageOwner"
4. Go to http://phorge.localhost/owners/edit/form/default/ and create a package named "TestPackageOwnerPackage" with the "Owner" set to "TestPackageOwner" (of course this is different from using http://phorge.localhost/packages/package/edit/1/ to create a package, God knows)
5. See that package also listed on http://phorge.localhost/owners/query/all/ as "Ox" (totally unneeded step I guess)
6. Go to http://phorge.localhost/maniphest/query/advanced/ and look at the placeholder text for the "Subscribers" field: `'Type a username, project, mailing list, package, or function...'`
7. In the "Subscribers" field, start typing the characters "TestPackageOwner"
8. In the "Subscribers" field dropdown, get the user and also "* Packages: TestPackageOwner" proposed
9. Go to http://phorge.localhost/applications/view/PhabricatorPackagesApplication/ and uninstall the "Packages" prototype application
10. Go to http://phorge.localhost/applications/view/PhabricatorOwnersApplication/ and uninstall the "Owners" application
11. Reload / go to http://phorge.localhost/maniphest/query/advanced/
12. Check the placeholder text for the "Subscribers" field
13. Start typing "TestPackageOwner" in the "Subscribers" field and check whether the dropdown proposals include "* Packages: TestPackageOwner" and/or "O1: TestPackageOwnerPackage"
14. Repeat steps 11 to 13 after (un)installing the Packages and/or Owners applications
Reviewers: O1 Blessed Committers, valerio.bozzolan, mainframe98
Reviewed By: O1 Blessed Committers, valerio.bozzolan, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16149
Differential Revision: https://we.phorge.it/D26140