packages: Mark the `inputs' field of <package> as thunked.

* guix/packages.scm (<package>)[inputs]: Mark as thunked.
  (package-derivation)[expand-input]: Remove case where the input is a
  procedure.
* tests/packages.scm ("trivial with system-dependent input"): Remove
  `lambda', and use (%current-system).
* gnu/packages/bootstrap.scm (package-from-tarball): Likewise for `inputs'.
  (%bootstrap-glibc, %bootstrap-gcc): Likewise.
* gnu/packages/scheme.scm (mit-scheme): Likewise.