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.md

@@ -11,7 +11,8 @@ General tips:
   [below](#secondary-dependencies) for more details.
 
 ## Identifiers
-Each package is required to define at least these variables:
+If package does not define a `$(package)_local_dir` variable, it is required to
+define these variables:
 
     $(package)_version:
     Version of the upstream library or program. If there is no version, a
@@ -28,6 +29,9 @@ Each package is required to define at least these variables:
     $(package)_sha256_hash:
     The sha256 hash of the upstream file
 
+If a package does define a `$(package)_local_dir` variable, the above variables
+are not required and will be ignored.
+
 These variables are optional:
 
     $(package)_build_subdir:
@@ -48,6 +52,18 @@ These variables are optional:
     Any extra files that will be fetched via $(package)_fetch_cmds. These are
     specified so that they can be fetched and verified via 'make download'.
 
+## Local packages
+
+If a package defines a `$(package)_local_dir` variable, the specified directory
+will be treated as a download source, and a tarball of its contents will be
+saved to `sources/`. A hash of the tarball will also become part of the package
+build id, so if the directory contents change, the package and everything
+depending on it will be rebuilt. For efficiency, the tarball is cached once it
+has been created, but if the local directory is touched, it will be rebuilt.
+
+Local packages can be useful for using git submodules or subtrees to manage
+package sources, or for testing local changes that are not available to
+download from an external source.
 
 ## Build Variables:
 After defining the main identifiers, build variables may be added or customized