* guix/ui.scm (%guix-user-module): New variable.
(read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.
guxi build: Allow gexps to be passed to '-e'.
* guix/ui.scm (%guix-user-module): New variable.
(read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.
* guix/monads.scm: Unexport 'lower-inputs' and 'derivation-expression'.
(text-file*): Add comment about the switch to 'gexp->derivation'.
(lower-inputs): Add comment about its doom.
(derivation-expression): Likewise.
* guix/gexp.scm (lower-inputs*): Rename to...
(lower-inputs): ... this. Update callers.
* tests/monads.scm (derivation-expression): New procedure.
* doc/guix.texi (The Store Monad): Use 'gexp->derivation' instead of
'derivation-expression'. Remove documentation of
'derivation-expression'.
* guix/ui.scm (read/eval): Use THE-ROOT-MODULE so that macros are
properly expanded.
* tests/guix-build.sh: Use 'gexp->derivation' instead of
'derivation-expression'.monads: Hide 'derivation-expression' and 'lower-inputs'.
* guix/ui.scm (read/eval): New procedure.
(read/eval-package-expression): Use it.
* guix/scripts/build.scm (derivations-from-package-expressions): Rename to...
(derivation-from-expression): ... this. Accept procedures, under the
assumption that they are monadic thunk.
(show-help): Adjust accordingly.
(guix-build): Ditto.
* tests/guix-build.sh: Add test.
* doc/guix.texi (Invoking guix build): Augment description of '-e'.
* guix-build.in (guix-build)[find-package]: New procedure.
Use it instead of using `find-packages-by-name' directly.
Suggested by Andreas Enge <andreas@enge.fr>.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix-build): Add `coreutils-8.20' as an
example. Fix guile-1.8 example.
* guix-build.in (derivations-from-package-expressions): Leave with an
error message when SOURCE? is #t and P has no source.
* tests/guix-build.sh: Add test.
