dhall: Use --base-import-url flag for documentation (#128588)

This updates `pkgs.dhallPackages.buildDhallGitHubPackage` to use the
newly added `--base-import-url` `dhall-docs` flag.  This flag is used
by the generated documentation so that paths copied to the clipboard
represent complete URLs that can be imported instead of only the
relative path to the import.

pkgs/development/interpreters/dhall/build-dhall-github-package.nix

@@ -43,7 +43,7 @@ lib.makePackageOverridable
         "rev"
       ]);
 
-      prefix = lib.optionalString (directory != "") "${directory}/";
+      prefix = lib.optionalString (directory != "") "/${directory}";
 
     in
       buildDhallPackage
@@ -51,9 +51,12 @@ lib.makePackageOverridable
 
             name = versionedName;
 
-            code = "${src}/${prefix}${file}";
+            code = "${src}${prefix}/${file}";
           }
         // lib.optionalAttrs document
-          { documentationRoot = "${src}/${prefix}"; }
+          { documentationRoot = "${src}/${prefix}";
+
+            baseImportUrl = "https://raw.githubusercontent.com/${owner}/${repo}/${rev}${prefix}";
+          }
         )
   )

pkgs/development/interpreters/dhall/build-dhall-package.nix

@@ -37,6 +37,12 @@
   #
   # If `null`, then no documentation is generated.
 , documentationRoot ? null
+
+  # Base URL prepended to paths copied to the clipboard
+  #
+  # This is used in conjunction with `documentationRoot`, and is unused if
+  # `documentationRoot` is `null`.
+, baseImportUrl ? null
 }:
 
 let
@@ -85,6 +91,12 @@ in
     ${lib.optionalString (documentationRoot != null) ''
     mkdir -p $out/${dataDhall}
 
-    XDG_DATA_HOME=$out/${data} ${dhall-docs}/bin/dhall-docs --input '${documentationRoot}' --package-name '${name}' --output-link $out/docs
+    XDG_DATA_HOME=$out/${data} ${dhall-docs}/bin/dhall-docs --output-link $out/docs ${lib.cli.toGNUCommandLineShell { } {
+      base-import-url = baseImportUrl;
+
+      input = documentationRoot;
+
+      package-name = name;
+    }}
     ''}
   ''