fetchDebianPatch: Handle the case of `lib*` packages

pkgs/build-support/fetchdebianpatch/default.nix

···
 lib.makeOverridable (
   { pname, version, debianRevision ? null, patch, hash,
     area ? "main", name ? "${patch}.patch" }:
-  let versionString =
-    if debianRevision == null then version else "${version}-${debianRevision}";
+  let
+    inherit (lib.strings) hasPrefix substring;
+    prefix =
+      substring 0 (if hasPrefix "lib" pname then 4 else 1) pname;
+    versionString =
+      if debianRevision == null then version
+      else "${version}-${debianRevision}";
   in fetchpatch {
     inherit name hash;
     url =
-      "https://sources.debian.org/data/${area}/${builtins.substring 0 1 pname}/"
+      "https://sources.debian.org/data/${area}/${prefix}/"
       + "${pname}/${versionString}/debian/patches/${patch}.patch";
   }
 )

pkgs/build-support/fetchdebianpatch/tests.nix

···
     patch = "Add-quotes-to-SOAPAction-header-in-SoapClient";
     hash = "sha256-xA8Wnrpr31H8wy3zHSNfezFNjUJt1HbSXn3qUMzeKc0=";
   };
+
+  libPackage = testers.invalidateFetcherByDrvHash fetchDebianPatch {
+    pname = "libfile-pid-perl";
+    version = "1.01";
+    debianRevision = "2";
+    patch = "missing-pidfile";
+    hash = "sha256-VBsIYyCnjcZLYQ2Uq2MKPK3kF2wiMKvnq0m727DoavM=";
+  };
 }