Introduces new router:
- `aurweb.routers.pkgbase`

Introduces new package:
- `aurweb.pkgbase`

Introduces new modules:
- `aurweb.pkgbase.actions`
- `aurweb.pkgbase.util`

Changes:
- `pkgbase_{action}_instance` functions are now located in
  `aurweb.pkgbase.actions`.
- `pkgbase`-wise routes have been moved to
  `aurweb.routers.pkgbase`.
- `make_single_context` was moved to
  `aurweb.pkgbase.util.make_context`.

Signed-off-by: Kevin Morris <kevr@0cost.org>