Allow Eshell variable aliases to point to other aliases

Allow Eshell variable aliases to point to other aliases

In particular, this resolves an issue where '$+' referenced the real
environment variable '$PWD' instead of the Eshell variable alias of
the same name.  This meant that changing directories in Eshell
wouldn't update the value of '$+'.

* lisp/eshell/esh-var.el (eshell-get-variable): Allow Eshell variable
aliaes to point to other aliases.

* test/lisp/eshell/em-dirs-tests.el (em-dirs-test/pwd-var)
(em-dirs-test/short-pwd-var): Adapt tests to check this case
(bug#56509).