Subcommand for migration from SDKv1 -> SDKv2

[kmoe]

Proposed name: v2upgrade

This will need to:

• Check for deprecations that cannot be automatically migrated

• Search and replace terraform.ResourceProvider type -> *schema.Provider type

• Search and replace validation functions in Remove deprecated validation funcs terraform-plugin-sdk#333

[kmoe]

Unfortunately the "search and replace" feature for replacing terraform.ResourceProvider with *schema.Provider is not as simple as it seems, as providers often use type assertions of the following form:

testAccProvider = Provider().(*schema.Provider)

(from https://github.com/hashicorp/terraform-provider-random/blob/386ee7e787caf841ad551b4f46d2819b6caa6f73/random/provider_test.go#L16)

[kmoe]

Sufficiently done.