Improve module usage with libroot and substrate

[leptos-null]

What does this implement/fix? Explain your changes.

• Change libroot.h from being a symlink to #include the header it previously linked to
  • this fixes a module build issue on clang 11
• Change substrate.h from being a copy of CydiaSubstrate/CydiaSubstrate.h to including it
  • Allows including substrate.h to use the CydiaSubstrate module
• remove link declaration from libroot/module.modulemap since the user may want to link against root or root_oldabi
  • this is not a breaking change, because theos automatically links root

Checklist

• New code follows the code rules
• I've added modulemaps for any new libraries (e.g. see libactivator/module.modulemap): it should be possible to @import MyLibrary; in ObjC, or import MyLibrary in Swift.
• My contribution is code written by myself from reverse-engineered headers, licensed into the Public Domain as per LICENSE.md; or, code written by myself / taken from an existing project, released under an OSI-approved license, and I've added relevant licensing credit to LICENSE.md

Does this close any currently open issues?

no

Any relevant logs, error output, etc?

Any other comments?

…

Where has this been tested?

Operating System: macOS

Platform: Darwin

Target Platform: iPhoneOS

Toolchain Version: Apple clang version 15.0.0 (clang-1500.3.9.4)

SDK Version: 16.5