Simplify creation of Eshell command forms

Simplify creation of Eshell command forms

Previously, Eshell over-aggressively converted subcommands, which forced
control flow commands to undo that change.

* lisp/eshell/esh-cmd.el (eshell-pre-rewrite-command-hook): Remove
'eshell-subcommand-arg-values' call from here...
(eshell-rewrite-named-command): ... and put it here.
(eshell-invokify-arg): Make obsolete.
(eshell-structure-basic-command): Handle subcommand test forms.
(eshell-rewrite-for-command, eshell-rewrite-while-command)
(eshell-rewrite-if-command): Don't call 'eshell-invokify-arg'; it's not
necessary.

(cherry picked from commit af029cdb3a69ba67ae88c9e93b7564778c61a3c6)