Remove relative path to vendored shards markd and reply

[nobodywasishere]

Following discussion in #13784, this PR (re)removes the relative paths to the vendored shards, allowing the Crystal compiler itself to be used as a library a lot easier.

This change does mean that in order to use the compiler within Crystal, it's necessary to add equivalent markd and reply shards to the project, as it won't be using the ones provided by the compiler itself.

Resolves #13784

[straight-shoota]

This change does mean that in order to use the compiler within Crystal, it's necessary to add equivalent markd and reply shards to the project, as it won't be using the ones provided by the compiler itself.

This is not entirely correct. If you're in the root directory of the crystal repo, the libraries are available in lib which is implictly available. When you're referencing the compiler source bundled with the Crystal package, this lib folder typically wouldn't be available anyway.
So this change doesn't have much impact in breaking setups that did work befor.