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

haskell#3708 says that an entry without an explicit exe component is shorthand for
all exe components, but this complicates parsing so I'm punting for now: an
explicit component name is required.

Cabal/Distribution/PackageDescription/Parse.hs

@@ -410,6 +410,9 @@ binfoFieldDescrs =
  , commaListField "build-tools"
  disp parse
  buildTools (\xs binfo -> binfo{buildTools=xs})
+ , commaListField "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

@@ -418,6 +418,9 @@ binfoFieldDescrs =
  , commaListField "build-tools"
  disp parsec
  buildTools (\xs binfo -> binfo{buildTools=xs})
+ , commaListField "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

@@ -28,6 +28,7 @@ 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
@@ -69,6 +70,7 @@ instance Monoid BuildInfo where
  mempty = BuildInfo {
  buildable = True,
  buildTools = [],
+ toolDepends = [],
  cppOptions = [],
  ccOptions = [],
  ldOptions = [],
@@ -104,6 +106,7 @@ 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,