Record flags for test component

At least one build system
[Haskell.nix](https://github.com/input-output-hk/haskell.nix) builds Haskell
projects a component at a time.  This breaks cabal-doctest's expected usage of
getting configuration from the Cabal components for libraries and executables
under test.

By recording flags for the test component, we introduce another possible usage
where all information we need comes from the test component alone.

See issue ulidtko#57 for more discussion and details.

src/Distribution/Extra/Doctest.hs

@@ -303,10 +303,7 @@ generateBuildModule testSuiteName flags pkg lbi = do
 
     additionalDirs <- mapM (fmap ("-i" ++) . makeAbsolute) additionalDirs'
 
-    -- Next, for each component (library or executable), we get to Build_doctests
-    -- the sets of flags needed to run doctest on that component.
-    let getBuildDoctests withCompLBI mbCompName compExposedModules compMainIs compBuildInfo =
-         withCompLBI pkg lbi $ \comp compCfg -> do
+    let recordComponent mbCompName compExposedModules compMainIs compBuildInfo comp compCfg = do
            let compBI = compBuildInfo comp
 
            -- modules
@@ -369,9 +366,12 @@ generateBuildModule testSuiteName flags pkg lbi = do
            -- modify IORef, append component
            modifyIORef componentsRef (\cs -> cs ++ [component])
 
-    -- For now, we only check for doctests in libraries and executables.
-    getBuildDoctests withLibLBI mbLibraryName           exposedModules (const Nothing)     libBuildInfo
-    getBuildDoctests withExeLBI (NameExe . executableName) (const [])     (Just . modulePath) buildInfo
+    -- record Build_doctests flags for test component
+    recordComponent (const $ NameExe testSuiteName) (const []) (const Nothing) testBuildInfo suite suitecfg
+    -- record Build_doctests flags for library component
+    withLibLBI pkg lbi $ recordComponent mbLibraryName exposedModules (const Nothing) libBuildInfo
+    -- record Build_doctests flags for executable component
+    withExeLBI pkg lbi $ recordComponent (NameExe . executableName) (const []) (Just . modulePath) buildInfo
 
     components <- readIORef componentsRef
     F.for_ components $ \(Component cmpName cmpPkgs cmpFlags cmpSources) -> do