Implement support for multiple Terraform version constraints from the module directory

[yermulnik]

When parsing Terraform version constraint from the module directory, only the first constraint found is taken into account, whilst Terraform configuration supports multiple terraform {} configuration blocks and each may have its own required_version parameter.
To improve Terraform versions detection the code needs to be adjusted to gather all the constraints and leverage combined constraint string to detect allowed required Terraform version.

Refs:

• Relevant existing code:

  terraform-switcher/main.go

  Line 443 in f268ab5

  tfconstraint := module.RequiredCore[0] //we skip duplicated definitions and use only first one

• Discussion: #356 (comment)
• Simplified implementation example:

  func installTFProvidedModule(dir string, custBinPath, mirrorURL *string) {
      fmt.Printf("Reading required version from terraform file\n")
      module, _ := tfconfig.LoadModule(dir)
      var tfconstraints []string
      for key := range module.RequiredCore {
          tfconstraints = append(tfconstraints, module.RequiredCore[key])
      }
      tfconstraint := strings.Join(tfconstraints, ", ")
      installFromConstraint(&tfconstraint, custBinPath, mirrorURL)
  }