command: Show data source reads differently in plans

Internally a data source read is represented as a creation diff for the
resource, but in the UI we'll show it as a distinct icon and color so that
the user can more easily understand that these operations won't affect
any real infrastructure.

Unfortunately by the time we get to formatting the plan in the UI we
only have the resource names to work with, and can't get at the original
resource mode. Thus we're forced to infer the resource mode by exploiting
knowledge of the naming scheme.

command/format_plan.go

@@ -96,6 +96,17 @@ func formatPlanModuleExpand(
 			color = "green"
 			symbol = "+"
 			oldValues = false
+
+			// If we're "creating" a data resource then we'll present it
+			// to the user as a "read" operation, so it's clear that this
+			// operation won't change anything outside of the Terraform state.
+			// Unfortunately by the time we get here we only have the name
+			// to work with, so we need to cheat and exploit knowledge of the
+			// naming scheme for data resources.
+			if strings.HasPrefix(name, "data.") {
+				symbol = "<="
+				color = "cyan"
+			}
 		case terraform.DiffDestroy:
 			color = "red"
 			symbol = "-"

command/plan.go

@@ -219,7 +219,7 @@ The Terraform execution plan has been generated and is shown below.
 Resources are shown in alphabetical order for quick scanning. Green resources
 will be created (or destroyed and then created if an existing resource
 exists), yellow resources are being changed in-place, and red resources
-will be destroyed.
+will be destroyed. Cyan entries are data sources to be read.
 
 Note: You didn't specify an "-out" parameter to save this plan, so when
 "apply" is called, Terraform can't guarantee this is what will execute.
@@ -230,7 +230,7 @@ The Terraform execution plan has been generated and is shown below.
 Resources are shown in alphabetical order for quick scanning. Green resources
 will be created (or destroyed and then created if an existing resource
 exists), yellow resources are being changed in-place, and red resources
-will be destroyed.
+will be destroyed. Cyan entries are data sources to be read.
 
 Your plan was also saved to the path below. Call the "apply" subcommand
 with this plan file and Terraform will exactly execute this execution