externals: Replace conan opus with git submodule

Apparently conan's prebuilt package ships without a runtime check for
AVX, crashing yuzu on devices without it.

Reverting this to externals seems to fix it. This wasn't tested on a CPU
without AVX, but it was tested with the following change on Windows:

bcdedit /set xsavedisable 1

master crashes inside opus code with an illegal instruction, this commit
doesn't.

externals/find-modules/Findopus.cmake

@@ -1,42 +0,0 @@
-
-find_package(PkgConfig QUIET)
-pkg_check_modules(PC_opus QUIET opus)
-
-find_path(opus_INCLUDE_DIR
-  NAMES opus.h
-  PATHS ${PC_opus_INCLUDE_DIRS}
-  PATH_SUFFIXES opus
-)
-find_library(opus_LIBRARY
-  NAMES opus
-  PATHS ${PC_opus_LIBRARY_DIRS}
-)
-
-include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(opus
-  FOUND_VAR opus_FOUND
-  REQUIRED_VARS
-    opus_LIBRARY
-    opus_INCLUDE_DIR
-  VERSION_VAR opus_VERSION
-)
-
-if(opus_FOUND)
-  set(Opus_LIBRARIES ${opus_LIBRARY})
-  set(Opus_INCLUDE_DIRS ${opus_INCLUDE_DIR})
-  set(Opus_DEFINITIONS ${PC_opus_CFLAGS_OTHER})
-endif()
-
-if(opus_FOUND AND NOT TARGET Opus::Opus)
-  add_library(Opus::Opus UNKNOWN IMPORTED)
-  set_target_properties(Opus::Opus PROPERTIES
-    IMPORTED_LOCATION "${opus_LIBRARY}"
-    INTERFACE_COMPILE_OPTIONS "${PC_opus_CFLAGS_OTHER}"
-    INTERFACE_INCLUDE_DIRECTORIES "${opus_INCLUDE_DIR}"
-  )
-endif()
-
-mark_as_advanced(
-    opus_INCLUDE_DIR
-    opus_LIBRARY
-)