* guix/packages.scm (<package-source>): Rename to...
(<origin>): ... this. Update users.
* distro/base.scm (libsigsegv, gawk, hello): Adjust to renaming; refer
to VERSION to build the URL.
* guix/packages.scm (package-derivation): When ARGS is a procedure, call
(ARGS SYSTEM).
* distro/base.scm (gawk)[arguments]: Turn into a procedure, to disable
tests on Cygwin.
* guix/packages.scm (<package>)[arguments]: Default to '().
* distro/base.scm (libsigsegv, gawk): New packages.
(hello): Use GAWK instead of (nixpkgs-derivation "gawk").
* guix/packages.scm (<location>): New record type.
(location, source-properties->location): New procedures.
(<package>)[location]: New field.
* tests/packages.scm ("GNU Hello"): Test `package-location'.
* Makefile.am (MODULES): Add `guix/packages.scm' and `distro/base.scm'.
(TESTS): Add `tests/packages.scm'.
(EXTRA_DIST): New variable.
* guix/packages.scm, distro/base.scm, tests/packages.scm: New files.
* guix/http.scm (http-fetch): Make `name' an optional argument, to match
the expectations of `package-source-derivation'.