Replace dependency on `unstable/lazy-require` with `racket/lazy-require`

[stamourv]

unstable/lazy-require is likely to go away in the future, and the functionality this package depends on is already provided by racket/lazy-require.

[samth]

What version of base does this need, for the info.rkt file?

[stamourv]

5.3.2 added racket/lazy-require, so that should be the one.

[samth]

Ah, then it doesn't matter so much, since packages were added then too. :)