emsdk

Properly report what is installed in "emsdk list" (#505)

This expands on #460 which added a file in each install dir
with the version, that lets us know not to re-download it. That
didn't integrate with is_installed, so it wasn't reported properly,
which this PR fixes.

With this we can remove a hack for python, as we can simply
not install anything if it's already installed (see line 1859).

Also add the "INSTALLED" indicators on the main listings in
"emsdk list".

Bug was reported in #477