packages: Convert source derivations to monadic style.

* guix/packages.scm (origin->derivation): Take body from
  'package-source-derivation', and change it to monadic style.  Expect
  METHOD to a monadic procedure.
  (package-source-derivation): Define in terms of 'origin->derivation'.
* guix/download.scm (url-fetch): Remove 'store' argument.  Remove
  'guile-for-build' variable.  Turn into a monadic procedure.
* guix/git-download.scm (git-fetch): Likewise.
* guix/svn-download.scm (svn-fetch): Likewise.
* tests/builders.scm (url-fetch*): New procedure.
  Change tests to call 'url-fetch*' instead of 'url-fetch'.
* tests/packages.scm ("package-source-derivation, snippet"): Remove
  'store' parameter of 'fetch' and change it to use 'interned-file'
  instead of 'add-to-store'.
* gnu/packages/bootstrap.scm (bootstrap-origin)[boot]: Remove 'store'
  parameter.