Make `sr-api-macros` and `srml-support` renaming of crates

Procedural do not support `$crate` to get access to the crate where the
macro is defined. We use a hack to re-export the crate under a known
name. With rust edition 2018, people started to rename their crates in
`Cargo.toml`, but that breaks the re-export. This commit introduces
`proc-maco-crate` that supports finding the requested crate name, even
if it was renamed.