Completely remove REVERSE_DEPS / deps_info using new wasm-ld feature

[sbc100]

Today in emscripten we have a reverse dependency system that is made up of two main parts:

1. Explicitly dependency info in deps_info.py and __deps entries in library files.
2. Scanning of all inputs using llvm-nm do discover which deps are needed.

With #18849 having been merged we can now move the deps information out of deps_info.py and into the library files.

I was planning on land that next phase, #18865, which enables transitive dependencies, but unfortunately that cause a regression because the current error message produced by wasm-ld was less useful than the jsifier ones:

Current wasm-ld error for missing reverse deps:

wasm-ld: error: symbol exported via --export not found: foo

vs:

error: undefined symbol: bar (referenced by foo__deps: ['bar'], referenced by emscripten_runtime_keepalive_pop__deps: ['foo','$runtimeKeepalivePop'], referenced by top-level compiled C/C++ code)

I think if we can make the wasm-ld message more like this then we could finally switch over to wasm-ld for all symbol resolution:

wasm-ld: error: undefined symbols `foo` required by `bar` which was imported from `libemscripten.txt`

This will require a new wasm-ld input format, which is kind of like a dummy shared library, but is essentially just a text file representing external symbol and their dependencies. My plan is to use a format much like the current current format we use for our cached symbol lists. e.g.:

js_sym
js_sym_foo: malloc, free

[sbc100]

I'm not sure why best to call this text file. We can think of it as a kind of linker script, or a dummy shared library.

[sbc100]

@dschuff pointer out some prior art here in terms of stub objects: https://docs.oracle.com/cd/E36784_01/html/E36857/chapter2-22.html

I used that term in my wasm-ld implementation: https://reviews.llvm.org/D145308