* guix/packages.scm (package-transitive-inputs): New procedure.
(package-derivation): Use it to compute INPUTS.
* tests/packages.scm (dummy-package): New macro.
("package-transitive-inputs"): New test.
* 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'.