support for performing `-upgrade=true` for root module with `-from-module`

### Terraform Version

```
$ terraform -v
Terraform v0.11.3
+ provider.dnsimple v0.1.0
+ provider.heroku v0.1.2
```

### Debug Output

https://gist.github.com/bernadinm/91900d553aa29de9585e1eed6a0f5bb0

### Expected Behavior

By running this command below, I expect it to update the root module locally and any other submodules.

```
terraform init -from-module github.com/hashicorp/terraform/examples//cross-provider -upgrade=true
```

### Actual Behavior

And error was printed out listed here: https://gist.github.com/bernadinm/91900d553aa29de9585e1eed6a0f5bb0
### Steps to Reproduce

1.
```
terraform init -from-module github.com/hashicorp/terraform/examples//cross-provider
```
2.
```
terraform init -from-module github.com/hashicorp/terraform/examples//cross-provider -upgrade=true
```