Change adding of .yaml extension in --defaults option.

Previously it was always added if the specified filename
lacked an extension. Now it is only added if there is
no file with the original name.  So, these are tried in
order: `FILE`, `FILE.yaml`, `DATADIR/defaults/FILE`,
`DATADIR/defaults/FILE.yaml`.

This facilitates using `--defaults` with bash/zsh `<(..)` or
`=(..)`.

Closes #11717.

MANUAL.txt

@@ -422,11 +422,12 @@ header when requesting a document from a URL:
     and output files, can be set using a defaults file.  The file will
     be searched for first in the working directory, and then in
     the `defaults` subdirectory of the user data directory
-    (see `--data-dir`).  The `.yaml` extension will be added if
-    *FILE* lacks an extension. See the section [Defaults files]
-    for more information on the file format. Settings from the
-    defaults file may be overridden or extended by subsequent
-    options on the command line.
+    (see `--data-dir`). If *FILE* lacks an extension, pandoc will
+    also try adding the `.yaml` extension if *FILE* is not found.
+    See the section [Defaults files] for more information on the
+    file format. Settings from the defaults file may be
+    overridden or extended by subsequent options on the command
+    line.
 
 `--bash-completion`
 

src/Text/Pandoc/App/Opt.hs

@@ -31,7 +31,7 @@ import Control.Monad.Except (throwError)
 import Control.Monad.Trans (MonadIO, liftIO, lift)
 import Control.Monad ((>=>), foldM)
 import Control.Monad.State.Strict (StateT, modify, gets)
-import System.FilePath ( addExtension, (</>), takeExtension, takeDirectory )
+import System.FilePath ( (<.>), (</>), takeDirectory )
 import System.Directory ( canonicalizePath )
 import Data.Char (toLower, isSpace)
 import Data.Maybe (fromMaybe)
@@ -891,19 +891,18 @@ fullDefaultsPath :: (PandocMonad m, MonadIO m)
                  -> FilePath
                  -> m FilePath
 fullDefaultsPath dataDir file = do
-  let fp = if null (takeExtension file)
-              then addExtension file "yaml"
-              else file
   defaultDataDir <- liftIO defaultUserDataDir
-  let defaultFp = fromMaybe defaultDataDir dataDir </> "defaults" </> fp
-  fpExists <- fileExists fp
-  if fpExists
-     then return fp
-     else do
-       defaultFpExists <- fileExists defaultFp
-       if defaultFpExists
-          then return defaultFp
-          else return fp
+  let ddir = fromMaybe defaultDataDir dataDir </> "defaults"
+  let findFile [] = return file
+      findFile (fp:fps) = do
+        fpExists <- fileExists fp
+        if fpExists
+           then return fp
+           else findFile fps
+  findFile [file,
+            file <.> "yaml",
+            ddir </> file,
+            ddir </> file <.> "yaml"]
 
 -- | In a list of lists, append another list in front of every list which
 -- starts with specific element.