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/BuildTreesitterParsers.cmake

@@ -1,5 +1,8 @@
 function(BuildTSParser LANG TS_URL TS_SHA256 TS_CMAKE_FILE)
   set(NAME treesitter-${LANG})
+  if(USE_EXISTING_SRC_DIR)
+    unset(TS_URL)
+  endif()
   ExternalProject_Add(${NAME}
     URL ${TS_URL}
     URL_HASH SHA256=${TS_SHA256}