Rename .cabal required-signatures field to signatures

Change of .cabal file syntax: rename @required-signatures@ field to
just @signatures@. Update the parser and error messages that mention
the field.

Also rename the corresponding field in the Library type.

Cabal/Distribution/PackageDescription/Check.hs

@@ -241,11 +241,11 @@ checkLibrary pkg lib =
                             Just n -> n
                             ) ++ "does not expose any modules"
 
-    -- check use of required-signatures/exposed-signatures sections
-  , checkVersion [1,21] (not (null (requiredSignatures lib))) $
+    -- check use of signatures sections
+  , checkVersion [1,25] (not (null (signatures lib))) $
       PackageDistInexcusable $
-           "To use the 'required-signatures' field the package needs to specify "
-        ++ "at least 'cabal-version: >= 1.21'."
+           "To use the 'signatures' field the package needs to specify "
+        ++ "at least 'cabal-version: >= 1.25'."
 
     -- check that all autogen-modules appear on other-modules or exposed-modules
   , check

Cabal/Distribution/PackageDescription/Parse.hs

@@ -177,8 +177,8 @@ libFieldDescrs =
   , commaListFieldWithSep vcat "reexported-modules" disp parse
       reexportedModules (\mods lib -> lib{reexportedModules=mods})
 
-  , listFieldWithSep vcat "required-signatures" disp parseModuleNameQ
-      requiredSignatures (\mods lib -> lib{requiredSignatures=mods})
+  , listFieldWithSep vcat "signatures" disp parseModuleNameQ
+      signatures (\mods lib -> lib{signatures=mods})
 
   , boolField "exposed"
       libExposed     (\val lib -> lib{libExposed=val})

Cabal/Distribution/Simple/Build.hs

@@ -416,7 +416,7 @@ testSuiteLibV09AsLibAndExe pkg_descr
             libName = Nothing,
             exposedModules = [ m ],
             reexportedModules = [],
-            requiredSignatures = [],
+            signatures = [],
             libExposed     = True,
             libBuildInfo   = bi
           }

Cabal/Distribution/Types/Library.hs

@@ -20,7 +20,7 @@ data Library = Library {
         libName :: Maybe String,
         exposedModules    :: [ModuleName],
         reexportedModules :: [ModuleReexport],
-        requiredSignatures:: [ModuleName], -- ^ What sigs need implementations?
+        signatures:: [ModuleName], -- ^ What sigs need implementations?
         libExposed        :: Bool, -- ^ Is the lib to be exposed by default?
         libBuildInfo      :: BuildInfo
     }
@@ -33,7 +33,7 @@ instance Monoid Library where
     libName = mempty,
     exposedModules = mempty,
     reexportedModules = mempty,
-    requiredSignatures = mempty,
+    signatures = mempty,
     libExposed     = True,
     libBuildInfo   = mempty
   }
@@ -44,7 +44,7 @@ instance Semigroup Library where
     libName = combine libName,
     exposedModules = combine exposedModules,
     reexportedModules = combine reexportedModules,
-    requiredSignatures = combine requiredSignatures,
+    signatures = combine signatures,
     libExposed     = libExposed a && libExposed b, -- so False propagates
     libBuildInfo   = combine libBuildInfo
   }
@@ -61,7 +61,7 @@ emptyLibrary = mempty
 explicitLibModules :: Library -> [ModuleName]
 explicitLibModules lib = exposedModules lib
               ++ otherModules (libBuildInfo lib)
-              ++ requiredSignatures lib
+              ++ signatures lib
 
 -- | Get all the auto generated module names from the library, exposed or not.
 -- This are a subset of 'libModules'.