gexp: Add '=>' syntax to import computed modules.

* guix/gexp.scm (imported-files)[file-pair]: Add case for pairs where
the cdr is not a string.
(imported-modules): Support '=>' syntax in MODULES.
* tests/gexp.scm ("imported-files with file-like objects")
("gexp->derivation & with-imported-module & computed module"): New tests.
* doc/guix.texi (G-Expressions): Document '=>' syntax for
'with-imported-modules'.