Handle multiple --source-map for stacks

[erickaby]

While using stacks there can be different sources throughout each stack and unit source. An example of this:

stack "example" {
  source = "git::git@github.com:example/catalog.git//stacks/example?ref=master"
}

# stack
unit "example" {
  source = "git::git@github.com:example/catalog.git//units/example?ref=master"
}

# unit
terraform {
  source = "tfr://github.com/example/aws/example?version=1.0.0"

  # locally at $HOME/catalog/modules/example
}

I want to run something like the below for local development.

terragrunt stack run plan \
    --source-map tfr://github.com/example=$HOME/catalog
    --source-map git::git@github.com:example/catalog.git=$HOME/catalog

# or

TG_SOURCE_MAP="git::git@github.com:example/catalog.git=$HOME/catalog tfr://github.com/example=$HOME/catalog"

Additional context

Follows these

• https://github.com/gruntwork-io/terragrunt-infrastructure-live-stacks-example
• https://github.com/gruntwork-io/terragrunt-infrastructure-catalog-example