This covers the case when a subcommand is to be invoked in more places
than before, for example when a subcommand is concatenated in an
argument.
* lisp/eshell/esh-cmd.el (eshell--find-subcommands): New fuction.
(eshell--invoke-command-directly): Use 'eshell-find-subcommands'.
* test/lisp/eshell/eshell-tests.el
(eshell-test/interp-cmd-external-concat): New test (bug#30725).