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
It was working previously by accident because the tests were
using the defaults such as `$HOME/.emscripten`. However usage of the
emsdk should really only happen in the emsdk environment where
EM_CONFIG dictates the config location (not just using the default).
This change is needed as precursor to making `--embedded` the default
(in this case the config doesn't live in `$HOME/.emscripten`).
