Make tfplugindocs location configurable with flag instead of PATH

Add a -tfplugindocsPath command line argument to the parsing script to
make it a bit more robust than dynamically setting the PATH variable in
the Makefile. Defaults to local bin, as the Makefile expects, but I
still set the argument in the Makefile in case someone modifies the BIN
variable.

GNUmakefile

@@ -57,9 +57,8 @@ $(BIN)/%:
 	@echo "Installing tools from tools/tools.go"
 	@cat tools/tools.go | grep _ | awk -F '"' '{print $$2}' | GOBIN=$(BIN) xargs -tI {} go install {}
 
-# Inject ./bin into PATH to allow scripts/generate-docs.go to access local tfplugindocs binary
 generate-docs: $(BIN)/tfplugindocs
-	PATH=$(PATH):$(BIN) go run scripts/generate-docs.go
+	go run scripts/generate-docs.go -tfplugindocsPath=$(BIN)/tfplugindocs
 
 validate-docs: $(BIN)/tfplugindocs
 	$(BIN)/tfplugindocs validate

scripts/generate-docs.go

@@ -27,6 +27,7 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"html/template"
 	"io"
@@ -57,6 +58,13 @@ type Page struct {
 }
 
 func main() {
+	tfplugindocsPath := flag.String("tfplugindocsPath", "bin/tfplugindocs", "location where tfplugindocs is installed")
+	flag.Parse()
+
+	if !tfPluginDocsExists(*tfplugindocsPath) {
+		log.Fatalf("tfplugindocs not found at '%s' - have you run the Makefile?", *tfplugindocsPath)
+	}
+
 	baseDir, err := os.Getwd()
 	if err != nil {
 		log.Fatal(err)
@@ -185,11 +193,19 @@ func main() {
 
 	replaceTemplatesDir(tmplDir, tempDir)
 
-	runTFPluginDocs()
+	runTFPluginDocs(*tfplugindocsPath)
 
 	replaceTemplatesDir(tmplDir, tmplDir+"-backup")
 }
 
+func tfPluginDocsExists(tfplugindocsLocation string) bool {
+	stat, err := os.Stat(tfplugindocsLocation)
+	if os.IsNotExist(err) {
+		return false
+	}
+	return !stat.IsDir()
+}
+
 // getTemplate walks the templates directory filtering non-tmpl extension
 // files, and parsing all the templates found (ensuring they must parse).
 func getTemplate(tmplDir string) *template.Template {
@@ -341,8 +357,8 @@ func replaceTemplatesDir(tmplDir string, tempDir string) {
 // consist of precompiled templates and that the original untouched templates
 // will still exist in the /templates-backup directory ready to be restored
 // once the /docs content has been generated.
-func runTFPluginDocs() {
-	cmd := exec.Command("tfplugindocs", "generate")
+func runTFPluginDocs(tfplugindocsLocation string) {
+	cmd := exec.Command(tfplugindocsLocation, "generate")
 	err := cmd.Run()
 	if err != nil {
 		log.Fatal(err)