internal/init: Package for helpers of the "terraform init" command

"terraform init" is quite a complex beast in relation to other commands
since almost everything it does is unique to it and thus not factored out
into other packages.

To get some of that sprawl out of the "command" package, this new internal
package will give us somewhere to put this init functionality that is
also useful for test code that needs to mimic the initialization behavior
against fixture directories.

internal/init/doc.go

@@ -0,0 +1,7 @@
+// Package init contains various helper functions used by the "terraform init"
+// command.
+//
+// These functions may also be used from testing code to simulate the behaviors
+// of "terraform init" against test fixtures, but should not be used elsewhere
+// in the main code.
+package init