Transform Links about readme-sync

[zheland]

Thank you for report.
I fixed the absolute path check function, so now the error will occur.
However, at the moment intra-doc links (like crate::Test) are not natively supported in pulldown-cmark, that is probably logical.
I added a function with which you can map links (see example in README.md) so you can now use intra-doc links with them separately specified in the readme sync test.

[daxpedda]

Things like std::clone::Clone aren't working either, thinking about it I can't figure out how they would have been supported anyway. std::clone::Clone has to be somehow converted to https://doc.rust-lang.org/core/clone/trait.Clone.html, I can't do that with map_links either, unless I do it manually for each intra-doc link.

So currently my solution will be to manually map_links all intra-doc links, but the next issue I encountered is this:

README.md:

[`Clone`](https://doc.rust-lang.org/core/clone/trait.Clone.html) text.

lib.rs:

//! [`Clone`] text.

I expected this to match, but it doesn't. My current workaround is:
README.md:

[`Clone`] text.

[`Clone`]: https://doc.rust-lang.org/core/clone/trait.Clone.html

lib.rs:

//! [`Clone`] text.
//!
//! [`Clone`]: Clone

I guess whats necessary is a function that removes all links, including link sections ([`Clone`]: Clone, I don't actually know the right terms) so it can match resolved links instead of where the link is specified.