fix: rewrite names of module declarations with relative paths

[schoel-bis]

This fixes #345.

Problem

Module declarations are just copied into the output file verbatim. This breaks for module declarations with relative paths. For example:

declare module "../config" {
  interface Config {
    foo: number;
  }
}

This will just simply be copied, although the relative module path will no longer exist after running this plugin.

This breaks cross-module type extensions with relative paths, i.e. extending the pre-existing Config interface from ../config.ts in the case above.

Solution

To solve this, a new fixer is introduced that rewrites all relative module declarations in such a way, that they reference the output file itself, i.e. if the output file were called index.d.ts, the above would be rewritten as:

declare module "./index" {
  interface Config {
    foo: number;
  }
}

Now, the pre-existing interface taken from ../config.ts will be properly extended again.