Breaking Change: Use `ghidra` instead of `sleigh` for header file directory

[ekilmer]

Now that upstream has added namespace ghidra to the source files, it probably makes more sense to place the header files in a layout like include/ghidra/*.hh.

This layout is still unofficial because upstream does not separate the headers.

This would be a breaking change because downstream users would be required to change the header include paths from include <sleigh/header.hh> to include <ghidra/header.hh>.

[ekilmer]

I also realized (while working on a PR for this) that it might make more sense to put Ghidra's upstream C++ code into include/ghidra but keep the extra support code and tools we distribute in our own include directory include/sleigh. This should help downstream users identify which parts of the codebase they depend on.