feat: add support for `upstream` directory in provider repo

[Thomas Meckel]

To resolve the documentation of a wrapped Terraform provider it's sometimes required to clone the Terraform provider repository into a directory named upstream into the repository of the Pulumi provider.

This following additions have to made for Makefile:

PULUMI_REPO_PATHS:= github.com/<tf-provider-repo>=$(ROOT_DIR)/upstream
upstream/.git:
        @echo "Initializing upstream" ; \
        git clone  --depth 1 --branch <tf-provider-version> https://github.com/<tf-provider-repo> upstream
tfgen:: install_plugins upstream/.git

Examples:

• pulumi-fortios: https://github.com/pulumiverse/pulumi-fortios/blob/8f2e8c138ab040096b7eebe587ce24f429f7c9c4/Makefile#L42
• pulumi-aws: https://github.com/pulumi/pulumi-aws/blob/e976921ff77a60ae528169351ca44ded4ecdee9c/Makefile#L135