Add "build-tool-depends" field to `BuildInfo` and parse it (with caveat)

A lack of ":exename" is susposed to indicate all exe components of the package,
but this complicates parsing so I'm punting for now: an explicit component name
is required.

Cabal/Distribution/PackageDescription/Parse.hs

@@ -418,6 +418,9 @@ binfoFieldDescrs =
  , commaListField  "build-tools"
            disp               parse
            buildTools         (\xs  binfo -> binfo{buildTools=xs})
+ , commaListField  "build-tool-depends"
+           disp               parse
+           toolDepends        (\xs  binfo -> binfo{toolDepends=xs})
  , commaListFieldWithSep vcat "build-depends"
            disp                   parse
            targetBuildDepends (\xs binfo -> binfo{targetBuildDepends=xs})

Cabal/Distribution/PackageDescription/Parsec/FieldDescr.hs

@@ -423,6 +423,9 @@ binfoFieldDescrs =
  , commaListField  "build-tools"
            disp               parsec
            buildTools         (\xs  binfo -> binfo{buildTools=xs})
+ , commaListField  "build-tool-depends"
+           disp               parsec
+           toolDepends        (\xs  binfo -> binfo{toolDepends=xs})
  , commaListFieldWithSep vcat "build-depends"
        disp               parsec
        targetBuildDepends (\xs binfo -> binfo{targetBuildDepends=xs})

Cabal/Distribution/Types/BuildInfo.hs

@@ -19,6 +19,7 @@ import Distribution.Compat.Prelude
 
 import Distribution.Types.Mixin
 import Distribution.Types.Dependency
+import Distribution.Types.ExeDependency
 import Distribution.Types.LegacyExeDependency
 import Distribution.Types.PkgconfigDependency
 
@@ -30,6 +31,7 @@ import Language.Haskell.Extension
 data BuildInfo = BuildInfo {
         buildable         :: Bool,      -- ^ component is buildable here
         buildTools        :: [LegacyExeDependency], -- ^ tools needed to build this bit
+        toolDepends       :: [ExeDependency], -- ^ haskell tools needed to build this bit
         cppOptions        :: [String],  -- ^ options for pre-processing Haskell code
         ccOptions         :: [String],  -- ^ options for C compiler
         ldOptions         :: [String],  -- ^ options for linker
@@ -71,6 +73,7 @@ instance Monoid BuildInfo where
   mempty = BuildInfo {
     buildable           = True,
     buildTools          = [],
+    toolDepends         = [],
     cppOptions          = [],
     ccOptions           = [],
     ldOptions           = [],
@@ -106,6 +109,7 @@ instance Semigroup BuildInfo where
   a <> b = BuildInfo {
     buildable           = buildable a && buildable b,
     buildTools          = combine    buildTools,
+    toolDepends         = combine    toolDepends,
     cppOptions          = combine    cppOptions,
     ccOptions           = combine    ccOptions,
     ldOptions           = combine    ldOptions,