* 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.