Makefile: replace enumer upstream with dmarkham's

Since the enumer implementation we used hadn't been updated for 5+
years, this didn't work with recent linux/go versions, and enumer
crashed while attempting to parse/analyse the source files.

There's another alternative on Github, forked from the one we used,
which seems more maintained now, and does produce the expected files in
the SDK.

Makefile

@@ -11,7 +11,7 @@ ci: testrace ## Test in continuous integration
 install-gen-deps: ## Install dependencies for code generation
 	@go install github.com/mna/[email protected]
 	# Pinning enumer at master branch; the latest tagged release is out of date.
-	@go install github.com/alvaroloes/enumer@master
+	@go install github.com/dmarkham/enumer@master
 
 	@go install github.com/hashicorp/packer-plugin-sdk/cmd/packer-sdc