gexp: 'imported-files' no longer creates a derivation by default.

gexp: 'imported-files' no longer creates a derivation by default.

* guix/gexp.scm (gexp->derivation): Add #:import-creates-derivation?.
Pass #:derivation? to 'imported-modules' and 'compiled-modules'.  In -L
argument, check whether MODULES is a derivation.
(%not-slash): New variable.
(file-mapping->tree): New procedure.
(imported-files): Rename to...
(imported-files/derivation): ... this.
(imported-files): New procedure.  Rewrite in terms of
'interned-file-tree' when possible; add #:derivation? parameter.
(imported-modules, compiled-modules): Add #:derivation? parameter and
pass it to 'imported-files'.
* guix/packages.scm (patch-and-repack): Pass
 #:import-creates-derivation? to 'gexp->derivation'.
* tests/gexp.scm ("imported-files"): Adjust to no longer expect a
derivation.