Enforce levelization in libxrpl with CMake (#5111)

Adds two CMake functions:

* add_module(library subdirectory): Declares an OBJECT "library" (a CMake abstraction for a collection of object files) with sources from the given subdirectory of the given library, representing a module. Isolates the module's headers by creating a subdirectory in the build directory, e.g. .build/tmp123, that contains just a symlink, e.g. .build/tmp123/basics, to the module's header directory, e.g. include/xrpl/basics, in the source directory, and putting .build/tmp123 (but not include/xrpl) on the include path of the module sources. This prevents the module sources from including headers not explicitly linked to the module in CMake with target_link_libraries.
* target_link_modules(library scope modules...): Links the library target to each of the module targets, and removes their sources from its source list (so they are not compiled and linked twice).

Uses these functions to separate and explicitly link modules in libxrpl:

    Level 01: beast
    Level 02: basics
    Level 03: json, crypto
    Level 04: protocol
    Level 05: resource, server

cmake/RippledCore.cmake

@@ -47,6 +47,57 @@ target_link_libraries(xrpl.libpb
     gRPC::grpc++
 )
 
+# TODO: Clean up the number of library targets later.
+add_library(xrpl.imports.main INTERFACE)
+target_link_libraries(xrpl.imports.main INTERFACE
+    LibArchive::LibArchive
+    OpenSSL::Crypto
+    Ripple::boost
+    Ripple::opts
+    Ripple::syslibs
+    absl::random_random
+    date::date
+    ed25519::ed25519
+    secp256k1::secp256k1
+    xxHash::xxhash
+)
+
+include(add_module)
+include(target_link_modules)
+
+# Level 01
+add_module(xrpl beast)
+target_link_libraries(xrpl.libxrpl.beast PUBLIC
+  xrpl.imports.main
+  xrpl.libpb
+)
+
+# Level 02
+add_module(xrpl basics)
+target_link_libraries(xrpl.libxrpl.basics PUBLIC xrpl.libxrpl.beast)
+
+# Level 03
+add_module(xrpl json)
+target_link_libraries(xrpl.libxrpl.json PUBLIC xrpl.libxrpl.basics)
+
+add_module(xrpl crypto)
+target_link_libraries(xrpl.libxrpl.crypto PUBLIC xrpl.libxrpl.basics)
+
+# Level 04
+add_module(xrpl protocol)
+target_link_libraries(xrpl.libxrpl.protocol PUBLIC
+  xrpl.libxrpl.crypto
+  xrpl.libxrpl.json
+)
+
+# Level 05
+add_module(xrpl resource)
+target_link_libraries(xrpl.libxrpl.resource PUBLIC xrpl.libxrpl.protocol)
+
+add_module(xrpl server)
+target_link_libraries(xrpl.libxrpl.server PUBLIC xrpl.libxrpl.protocol)
+
+
 add_library(xrpl.libxrpl)
 set_target_properties(xrpl.libxrpl PROPERTIES OUTPUT_NAME xrpl)
 if(unity)
@@ -60,10 +111,24 @@ file(GLOB_RECURSE sources CONFIGURE_DEPENDS
 )
 target_sources(xrpl.libxrpl PRIVATE ${sources})
 
-target_include_directories(xrpl.libxrpl
-  PUBLIC
-    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-    $<INSTALL_INTERFACE:include>)
+target_link_modules(xrpl PUBLIC
+  basics
+  beast
+  crypto
+  json
+  protocol
+  resource
+  server
+)
+
+# All headers in libxrpl are in modules.
+# Uncomment this stanza if you have not yet moved new headers into a module.
+# target_include_directories(xrpl.libxrpl
+#   PRIVATE
+#     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
+#   PUBLIC
+#     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+#     $<INSTALL_INTERFACE:include>)
 
 target_compile_definitions(xrpl.libxrpl
   PUBLIC