Refactor `Unit` related packets with `SpellEntry` #240

Job	Run time
Build on Ubuntu GCC (client build 4878)	5m 56s
Build on Ubuntu GCC (client build 5302)	6m 57s
Build on Ubuntu GCC (client build 4695)	7m 4s
Build on Ubuntu GCC (client build 5464)	7m 14s
Build on Ubuntu GCC (client build 5086)	7m 8s
Build on Ubuntu GCC (client build 4544)	7m 2s
Build on Ubuntu GCC (client build 5875)	7m 13s
Build on Ubuntu Clang (client build 5875)	4m 13s
Build on Windows MinGW (client build 5875)	10m 27s
Build on Windows Visual Studio (client build 5875)	5m 38s
Build on Ubuntu GCC (client build 4375)	6m 42s
Build on Ubuntu GCC (client build 4449)	6m 53s
Build on macOS Clang (client build 5875)	5m 52s
	1h 28m 19s

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