lib: add conditional dep to package.yaml and rest of cabal file

hledger-lib/hledger-lib.cabal

@@ -2,7 +2,7 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 0dda6f4ef72b1a815284abd47c8dfefe75812e2e91900581bd7837dd587533d4
+-- hash: 9878d76090f5076d1d95f3f77f92ef1683e98b5567dfcd1613a225f85ece8dfe
 
 name:           hledger-lib
 version:        1.5.99
@@ -84,8 +84,9 @@ library
     , transformers >=0.2
     , uglymemo
     , utf8-string >=0.3.5
-  if !impl(ghc >= 8.0)
+  if (!impl(ghc >= 8.0))
-    build-depends: semigroups == 0.18.*
+    build-depends:
+        semigroups ==0.18.*
   exposed-modules:
       Hledger
       Hledger.Data
@@ -177,6 +178,9 @@ test-suite doctests
     , transformers >=0.2
     , uglymemo
     , utf8-string >=0.3.5
+  if (!impl(ghc >= 8.0))
+    build-depends:
+        semigroups ==0.18.*
   other-modules:
       Hledger
       Hledger.Data
@@ -267,6 +271,9 @@ test-suite easytests
     , transformers >=0.2
     , uglymemo
     , utf8-string >=0.3.5
+  if (!impl(ghc >= 8.0))
+    build-depends:
+        semigroups ==0.18.*
   other-modules:
       Hledger
       Hledger.Data
@@ -358,6 +365,9 @@ test-suite hunittests
     , transformers >=0.2
     , uglymemo
     , utf8-string >=0.3.5
+  if (!impl(ghc >= 8.0))
+    build-depends:
+        semigroups ==0.18.*
   other-modules:
       Hledger
       Hledger.Data

hledger-lib/package.yaml

@@ -76,6 +76,11 @@ dependencies:
 #- system-filepath
 #- trifecta >=0.91
 
+when:
+- condition: (!impl(ghc >= 8.0))
+  dependencies:
+  - semigroups == 0.18.*
+
 ghc-options:
 - -Wall
 - -fno-warn-unused-do-bind