Avoid loot-prelude dependency

Problem: lootbox is not uploaded to hackage, so when we upload
xrefcheck, its build on hackage fails.

Solution: we can use `mixins` feature to set Universum as prelude
without any intermediate packages.

This seems to be supported even by quite old versions of stack, like
1.7.3, so that should not bring build problems in most cases.

package.yaml

@@ -2,6 +2,8 @@
 #
 # SPDX-License-Identifier: MPL-2.0
 
+spec-version: 0.31.0
+
 name:                xrefcheck
 version:             0.1.1
 github:              serokell/xrefcheck
@@ -73,7 +75,6 @@ dependencies:
   - http-client
   - http-types
   - lens
-  - loot-prelude
   - pretty-terminal
   - mtl
   - o-clock
@@ -85,6 +86,8 @@ dependencies:
   - text-metrics
   - th-lift-instances
   - th-utilities
+  - name: universum
+    mixin: [(Universum as Prelude), (Universum.Unsafe as Unsafe)]
   - yaml
   - with-utf8
 

stack.yaml

@@ -13,8 +13,3 @@ extra-deps:
 - aeson-options-0.1.0
 - o-clock-1.1.0
 - with-utf8-1.0.0.0
-
-- git: https://github.com/serokell/lootbox.git
-  commit: 18ced0d493348b4d6bae8517dff7ee6de6e0c9d9 # master
-  subdirs:
-    - code/prelude

stack.yaml.lock

@@ -33,21 +33,12 @@ packages:
   original:
     hackage: o-clock-1.1.0
 - completed:
-    subdir: code/prelude
-    cabal-file:
-      size: 898
-      sha256: 57621e886108b62e43236c4a9d562a0330a714188193a9cc50ced8024bd138eb
-    name: loot-prelude
-    version: 0.0.0.0
-    git: https://github.com/serokell/lootbox.git
+    hackage: with-utf8-1.0.0.0@sha256:686e47588986d8080451b4e617118b579487dd4e085bba7bb36fac4198c90ae6,2480
     pantry-tree:
-      size: 264
-      sha256: 47da19ed2f317b2f7c6f70a8517c275e7bd8bf25bd628cf271d08d6b5bc5b8ec
-    commit: 18ced0d493348b4d6bae8517dff7ee6de6e0c9d9
+      size: 905
+      sha256: 39176872f0dde9f9e09c9cb9496e2b7b10fa17cb9a6eca8d40ca4b2dcaaacc11
   original:
-    subdir: code/prelude
-    git: https://github.com/serokell/lootbox.git
-    commit: 18ced0d493348b4d6bae8517dff7ee6de6e0c9d9
+    hackage: with-utf8-1.0.0.0
 snapshots:
 - completed:
     size: 524804