Search for metadata files in $DATADIR/metadata (#7851)

If files specified with `--metadata-file` are not found in the working
directory, look in `$DATADIR/metadata`.

Expose new `readMetadataFile` function from Text.Pandoc.Class
[API change].

Expose new `PandocCouldNotFindMetadataFileError` constructor for
`PandocError` from Text.Pandoc.Error [API change].

Closes #5876.

4 files changed, 67 insertions, 0 deletions

diff --git a/test/command/5876.md b/test/command/5876.md
new file mode 100644
index 000000000..7671a89cf
--- /dev/null
+++ b/test/command/5876.md
@@ -0,0 +1,58 @@
+```
+% pandoc -s -t native --data-dir=command/5876 --metadata-file=5876.yaml
+Hello
+^D
+Pandoc
+  Meta
+    { unMeta =
+        fromList
+          [ ( "desc"
+            , MetaInlines
+                [ Str "Found"
+                , Space
+                , Str "in"
+                , Space
+                , Str "metadata"
+                , Space
+                , Str "directory."
+                ]
+            )
+          ]
+    }
+  [ Para [ Str "Hello" ] ]
+```
+```
+% pandoc -s -t native --data-dir=command/5876 --metadata-file=command/5876.yaml
+Hello
+^D
+Pandoc
+  Meta
+    { unMeta =
+        fromList
+          [ ( "desc"
+            , MetaInlines
+                [ Str "Found"
+                , Space
+                , Str "in"
+                , Space
+                , Str "working"
+                , Space
+                , Str "directory."
+                ]
+            )
+          ]
+    }
+  [ Para [ Str "Hello" ] ]
+```
+```
+% pandoc -s -t native --data-dir=command/5876 --metadata-file=does-not-exist.yaml
+Hello
+^D
+Could not find metadata file does-not-exist.yaml
+```
+```
+% pandoc -s -t native --metadata-file=does-not-exist.yaml
+Hello
+^D
+Could not find metadata file does-not-exist.yaml
+```
diff --git a/test/command/5876.yaml b/test/command/5876.yaml
new file mode 100644
index 000000000..9e6604922
--- /dev/null
+++ b/test/command/5876.yaml
@@ -0,0 +1,3 @@
+---
+desc: Found in working directory.
+---
diff --git a/test/command/5876/metadata/5876.yaml b/test/command/5876/metadata/5876.yaml
new file mode 100644
index 000000000..d52de910f
--- /dev/null
+++ b/test/command/5876/metadata/5876.yaml
@@ -0,0 +1,3 @@
+---
+desc: Found in metadata directory.
+---
diff --git a/test/command/5876/metadata/command/5876.yaml b/test/command/5876/metadata/command/5876.yaml
new file mode 100644
index 000000000..6bb46d51e
--- /dev/null
+++ b/test/command/5876/metadata/command/5876.yaml
@@ -0,0 +1,3 @@
+---
+desc: Should not be loaded.
+---