caller_crate_root() is wrong in some very specific edge case

[sam0x17]

Originally reported here: paritytech/substrate#14672 (comment)

What's telling is this:

1. macro invocation site: frame/paged-list/src/lib.rs
2. caller_crate_root() at that site: frame/paged-list/fuzzer
3. CARGO_MANIFEST_DIR at that site: frame/paged-list
4. CARGO_CRATE_NAME at that site: pallet_paged_list
5. CARGO_PKG_NAME at that site: pallet-paged-list

So somehow caller_crate_root() is failing to explore the parent directory in this very specific situation that we can only reproduce in CI

[sam0x17]

So the reason for this is actually quite simple.

The code that looks for the crate_root looks for Cargo.toml files where name = "{{crate_name}}" is in the file. Normally this is an OK assumption but the fuzzer has this line in the bin section:

[[bin]]
name = "pallet-paged-list"

So my assumption is bad and what I should be doing is real TOML parsing here. So will add that.