This change eliminates our dependencies on the SDK repo. Now that
SDKs are comprised solely of pulumi/pulumi artifacts, a separate repo
isn't required. This allows us to simplify some of the distribution.
The install.sh script is modified slightly, to permit overriding the
default install location using $PULUMI_INSTALL_PATH.
After the move to stop including packages in the SDK, we no longer
published an SDK per build. This corrects this. Since the only things
in the SDK today are the language plugins and the CLI itself, we can
publish an SDK per build from pulumi/pulumi.
This change re-uses the existing infrastructure we have in
pulumi/sdk.
Fixes#1076
