LOCRDT y-crdt: bundle as external

Unfortunately it has 144 dependencies so hack Makefile.fetch to invoke
cargo fetch to get them all at once, based on the Cargo.lock file
extracted from the tarball; this all ends up in a "cargo" subdirectory
below $TARFILE_LOCATION.

Then $CARGO_HOME must be set consistently in the ExternalProject file
and cargo invoked with --offline to prevent downloading during the
build; altogether uses about 720 MB of storage here.

Change-Id: I6126fdd988f12bf9da48b6c2d9123d349eb7cb96
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/186903
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
Tested-by: Jenkins

RepositoryExternal.mk

@@ -4466,11 +4466,12 @@ endif
 ifneq ($(WITH_YRS),)
 
 define gb_LinkTarget__use_yrs
+$(call gb_LinkTarget_use_external_project,$(1),y-crdt)
 $(call gb_LinkTarget_set_include,$(1),\
 	$$(INCLUDE) \
-	-I$(WITH_YRS)/tests-ffi/include \
+	-I$(gb_UnpackedTarball_workdir)/y-crdt/tests-ffi/include \
 )
-$(call gb_LinkTarget_add_libs,$(1),$(WITH_YRS)/target/debug/libyrs.a)
+$(call gb_LinkTarget_add_libs,$(1),$(gb_UnpackedTarball_workdir)/y-crdt/target/debug/libyrs.a)
 endef
 
 else