Refactor folder (#10475)

Move many third-party components' source codes from the src folder to a new folder called deps_src. The goal is to make the code structure clearer and easier to navigate.

deps_src/eigen/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.13)
+project(eigen)
+
+add_library(eigen INTERFACE)
+
+target_include_directories(eigen INTERFACE
+    ${CMAKE_CURRENT_SOURCE_DIR}
+)
+
+# Eigen is header-only, so we only need to specify the include directory
+# The headers are in the Eigen/ subdirectory structure