derivation: Coalesce multiple occurrences of the same input.

* guix/derivations.scm (write-derivation)[coalesce-duplicate-inputs]:
  New procedure.
  Use it to process INPUTS.

* tests/derivations.scm ("user of multiple-output derivation"): New
  test.

Fix `derivation-hash' for outputs not sorted alphabetically.

* guix/derivations.scm (derivation-hash): Reorder OUTPUTS.
* tests/derivations.scm ("multiple-output derivation, non-alphabetic
  order"): New test.

Add `derivation-prerequisites' and `derivation-prerequisites-to-build'.

* guix/derivations.scm (derivation-prerequisites,
  derivation-prerequisites-to-build): New procedures.

* tests/derivations.scm ("build-expression->derivation and
  derivation-prerequisites", "build-expression->derivation and
  derivation-prerequisites-to-build"): New tests.

Add the `valid-path?' RPC.

* guix/store.scm (valid-path?): New procedure.

* tests/builders.scm ("http-fetch", "gnu-build"): Use it.
* tests/derivations.scm ("add-to-store, flat", "add-to-store,
  recursive", "derivation with no inputs", "build derivation with 1
  source", "build derivation with coreutils",
  "build-expression->derivation with expression returning #f"):
  Likewise.

build-expression->derivation: Have the build fail when EXP returns #f.

* guix/derivations.scm (build-expression->derivation)[builder]: Pass the
  result of EXP to `exit'.

* tests/derivations.scm ("build-expression->derivation with expression
  returning #f"): New test.

Change `build-expression->derivation' to support sub-derivation inputs.

* guix/derivations.scm (build-expression->derivation): Change to expect
  INPUTS to have the form (NAME DRV-PATH SUB-DRV) or (NAME DRV-PATH),
  instead of (NAME . DRV-PATH).  Update callers accordingly.

* guix/gnu-build-system.scm, tests/builders.scm, tests/derivations.scm:
  Update accordingly.

Fix `imported-files' for files with a common prefix.

* guix/derivations.scm (imported-files)[builder]: For TAIL, check
  whether it exists before (mkdir TAIL).

* tests/derivations.scm ("imported-files"): Add file with a common
  prefix.

Add `add-to-store' test for flat files.

* tests/derivations.scm ("add-to-store, flat"): New test.

Add `imported-files'.

* guix/derivations.scm (imported-files): New procedure.
  (build-expression->derivation): Correctly handle inputs that are
  sources and not derivation paths.

* tests/derivations.scm ("imported-files"): New test.

Fix the `build-expression->derivation' fixed-output test.

* tests/derivations.scm ("build-expression->derivation"): Fix the
  builder to actually pass a bytevector to `put-bytevector'.

Add a `%current-system' fluid.

* guix/utils.scm (gnu-triplet->nix-system): New procedure.
  (%current-system): New variable.

* tests/utils.scm ("gnu-triplet->nix-system"): New test.

* tests/derivations.scm (%current-system): Remove.  Update users to
  use (%current-system) instead.

Add `add-to-store' with recursive directory storage.

* guix/store.scm (write-file): Implement directory recursive dump.
  (add-to-store): Fix the parameter list.

* tests/derivations.scm (directory-contents): New procedure.
  ("add-to-store, recursive"): New test.

tests: Don't hard-code the system type.

* tests/derivations.scm (%current-system): New variable.  Replace
  occurrences of "x86_64-linux" by %CURRENT-SYSTEM.

Add multiple-output support to `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Add `outputs'
  keyword parameter; pass it to `derivation'.  Define `%outputs' in the
  prologue.

* tests/derivations.scm ("build-expression->derivation with two
  outputs"): New test.

Add `derivation' test for multiple-output derivations.

* tests/derivations.scm ("multiple-output derivation"): New test.

Sort environment variables alphabetically in `derivation'.

* guix/derivations.scm (derivation)[env-vars-with-empty-outputs]: Sort
  the result alphabetically.

* tests/derivations.scm ("build derivation with 1 source"): Add
  environment variables.

Add support for fixed-output derivations in `build-expression->derivation'.

* guix/derivations.scm (build-expression->derivation): Pass HASH and
  HASH-ALGO to `derivation'.

* tests/derivations.scm ("build-expression->derivation for fixed-output
  derivation"): New test.

Add support for fixed-output derivations.

* guix/derivations.scm (read-derivation)[outputs->alist]: For
  fixed-outputs, convert HASH with `base16-string->bytevector'.
  (write-derivation): Likewise, convert HASH-ALGO to a string and HASH
  to a base16 string.
  (derivation-hash): Expect HASH to be a bytevector, not a string;
  convert HASH with `bytevector->base16-string'.

* tests/derivations.scm ("fixed-output derivation"): New test.

Add `build-expression->derivation'.

* guix/derivations.scm (%guile-for-build): New parameter.
  (build-expression->derivation): New procedure.

* tests/derivations.scm ("build-expression->derivation without inputs",
  "build-expression->derivation with one input"): New tests.

Allow derivations with input derivations.

* guix/derivations.scm (derivation-path->output-path): New procedure.
  (derivation-hash): Call `memoize'.  In the fixed-output case, convert
  HASH-ALGO to a string.  In the other case, sort inputs in the
  alphabetical order of their hex hash.  For inputs with no sub-drvs,
  add "out" as the sub-drv.

* guix/utils.scm (%nixpkgs-directory): New parameter.
  (nixpkgs-derivation, memoize): New procedures.

* tests/derivations.scm ("build derivation with 1 source"): Remove
  useless shebang.
  (%coreutils): New variable.
  ("build derivation with coreutils"): New test.