build(externals): rename Findopus to FindOpus

This better matches upstream's FindOpus.cmake file, and it will make
using upstream's FindOpus.cmake file easier.

externals/find-modules/FindOpus.cmake

@@ -0,0 +1,19 @@
+# SPDX-FileCopyrightText: 2022 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+find_package(PkgConfig)
+
+if (PKG_CONFIG_FOUND)
+    pkg_search_module(opus IMPORTED_TARGET GLOBAL opus)
+    if (opus_FOUND)
+        add_library(Opus::opus ALIAS PkgConfig::opus)
+    endif()
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Opus
+    REQUIRED_VARS
+        opus_LINK_LIBRARIES
+        opus_FOUND
+    VERSION_VAR opus_VERSION
+)