* guix/gexp.scm (<scheme-file>)[splice?]: New field.
(scheme-file): Add #:splice? and pass it to '%scheme-file'.
(scheme-file-compiler): Pass SPLICE? to 'gexp->file'.
(gexp->file): Add #:splice? and honor it.
* tests/gexp.scm ("gexp->file + #:splice?"): New test.
("gexp->derivation & with-imported-module & computed module"): Use
#:splice? #t.