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/GetBinaryDeps.cmake

@@ -22,6 +22,9 @@ function(GetBinaryDep)
     message(FATAL_ERROR "${URL_VARNAME} and ${HASH_VARNAME} must be set")
   endif()
 
+  if(USE_EXISTING_SRC_DIR)
+    unset(URL)
+  endif()
   ExternalProject_Add(${_gettool_TARGET}
     URL ${URL}
     URL_HASH SHA256=${HASH}