inferior: 'gexp->derivation-in-inferior' honors EXP's load path.

Previously the imported modules and extensions of EXP would be missing
from the load path of 'guix repl'.

* guix/inferior.scm (gexp->derivation-in-inferior)[script]: New
variable.
[trampoline]: Write (primitive-load #$script) to PIPE.  Add #$output.
* tests/channels.scm ("channel-instances->manifest")[depends?]: Check
for requisites rather than direct references.
Adjust callers accordingly.