depends: Switch libmultiprocess packages to use local git subtree

With newly introduced libmultiprocess subtree, there's no need for depends
system to download and track changes to the upstream repository.

Note that adding the libmultiprocess subtree does not allow dropping
libmultiprocess packages from the depends build, because libmultiprocess
includes a code generation tool called mpgen, and in cross-compiled builds,
bitcoin core's cmake build system doesn't have access to a native toolchain and
can't build mpgen itself, so the depends system (or the native environment if
not using depends) needs to supply it.

depends/packages/native_libmultiprocess.mk

@@ -1,8 +1,5 @@
 package=native_libmultiprocess
-$(package)_version=35944ffd23fa26652b82210351d50e896ce16c8f
-$(package)_download_path=https://github.com/bitcoin-core/libmultiprocess/archive
-$(package)_file_name=$($(package)_version).tar.gz
-$(package)_sha256_hash=b542f270c076d0287c124e7f97b21ab7e32b979ff182491c157c2da9dec723c4
+$(package)_local_dir=../src/ipc/libmultiprocess
 $(package)_dependencies=native_capnp
 
 define $(package)_config_cmds