build(deps): restore support for USE_EXISTING_SRC_DIR (#20491)

59d5f692f removed cmake.deps/cmake/DownloadAndExtractFile.cmake and
support for USE_EXISTING_SRC_DIR.  The Ubuntu nightly PPA still relies
on USE_EXISTING_SRC_DIR functionality since it can't access the network
during the build.

Supplying an empty value for ExternalProject_Add()'s URL value appears
to provide the needed mechanism to avoid re-downloading when the sources
are already present.  This is undocumented behavior, though, so it may
break in the future.

Now, if USE_EXISTING_SRC_DIR is set, the ExternalProject's URL variable
is unset, preventing the download and erroring out if the source doesn't
actually exist.

cmake.deps/cmake/BuildMsgpack.cmake

@@ -21,6 +21,9 @@ if(MSVC)
     -DCMAKE_GENERATOR=${CMAKE_GENERATOR})
 endif()
 
+if(USE_EXISTING_SRC_DIR)
+  unset(MSGPACK_URL)
+endif()
 ExternalProject_Add(msgpack
   URL ${MSGPACK_URL}
   URL_HASH SHA256=${MSGPACK_SHA256}