core: expose terraform.ModuleTreeDependencies as a public function

This is a generally-useful utility for computing dependency trees, so no
reason to restrict it to just the terraform package.

terraform/context.go

@@ -178,7 +178,7 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 	var providers map[string]ResourceProviderFactory
 	if opts.ProviderResolver != nil {
 		var err error
-		deps := moduleTreeDependencies(opts.Module, state)
+		deps := ModuleTreeDependencies(opts.Module, state)
 		reqd := deps.AllPluginRequirements()
 		providers, err = resourceProviderFactories(opts.ProviderResolver, reqd)
 		if err != nil {

terraform/module_dependencies.go

@@ -7,7 +7,7 @@ import (
 	"github.com/hashicorp/terraform/plugin/discovery"
 )
 
-// moduleTreeDependencies returns the dependencies of the tree of modules
+// ModuleTreeDependencies returns the dependencies of the tree of modules
 // described by the given configuration tree and state.
 //
 // Both configuration and state are required because there can be resources
@@ -16,7 +16,7 @@ import (
 // This function will panic if any invalid version constraint strings are
 // present in the configuration. This is guaranteed not to happen for any
 // configuration that has passed a call to Config.Validate().
-func moduleTreeDependencies(root *module.Tree, state *State) *moduledeps.Module {
+func ModuleTreeDependencies(root *module.Tree, state *State) *moduledeps.Module {
 
 	// First we walk the configuration tree to build the overall structure
 	// and capture the explicit/implicit/inherited provider dependencies.

terraform/module_dependencies_test.go

@@ -248,7 +248,7 @@ func TestModuleTreeDependencies(t *testing.T) {
 				root = testModule(t, test.ConfigDir)
 			}
 
-			got := moduleTreeDependencies(root, test.State)
+			got := ModuleTreeDependencies(root, test.State)
 			if !got.Equal(test.Want) {
 				t.Errorf(
 					"wrong dependency tree\ngot:  %s\nwant: %s",