feat: allow generate command to generate a single DBC file

This commit allows a user to run

    cantool generate INPUT OUTPUT-DIRECTORY

where INPUT is either a path to a DBC file. Previously it was only
possible to provide INPUT as a directory containing DBC files.

N.B. When INPUT is a directory containing DBC files the generated
files are created in OUTPUT-DIRECTORY relative to where they are found
in INPUT.

As an example, given the following directory structure:

    dbc/
       powertrain/pt.dbc
       steering/steer.dbc
       brake/brake.dbc

A `cantool generate dbc/ gen/go` call will generate the following
structure:

    gen/go/
       powertrain/pt.dbc.go
       steering/steer.dbc.go
       brake/brake.dbc.go

However, when INPUT is a single DBC file it is generated directly in
OUTPUT-DIRECTORY with its basename as the stem.

`cantool generate dbc/steering/steer.dbc gen/go/steering` will
therefore generate the gen/go/steering/steer.dbc.go.

Author: Jassob

README.md

@@ -75,13 +75,52 @@ func main() {
 It is possible to generate Go code from a `.dbc` file.
 
 ```
-$ go run go.einride.tech/can/cmd/cantool generate <dbc file root folder> <output folder>
+$ go run go.einride.tech/can/cmd/cantool generate <input> <output folder>
 ```
 
+The INPUT argument can be either a directory containing DBC files, or the path
+to a single DBC file.
+
+#### Placement of generated code
+
+OUTPUT FOLDER is treated slightly different dependeing on whether INPUT is a
+file or a directory.
+
+When INPUT is a directory containing DBC files the generated files are created
+in OUTPUT-DIRECTORY relative to where they are found in INPUT.
+
+As an example, given the following directory structure:
+
+```
+dbc/
+   powertrain/pt.dbc
+   steering/steer.dbc
+   brake/brake.dbc
+```
+
+A `cantool generate dbc/ gen/go` call will generate the following structure:
+
+```
+gen/go/
+   powertrain/pt.dbc.go
+   steering/steer.dbc.go
+   brake/brake.dbc.go
+```
+
+When INPUT is a single DBC file it is generated directly in OUTPUT-DIRECTORY
+with its basename as the stem.
+
+`cantool generate dbc/steering/steer.dbc gen/go/steering` will therefore
+generate the gen/go/steering/steer.dbc.go.
+
+#### Linting
+
 In order to generate Go code that makes sense, we currently perform some
 validations when parsing the DBC file so there may need to be some changes on
 the DBC file to make it work
 
+#### Using generated code
+
 After generating Go code we can marshal a message to a frame:
 
 ```go

cmd/cantool/main.go

@@ -44,26 +44,30 @@ func main() {
 
 func generateCommand(app *kingpin.Application) {
 	command := app.Command("generate", "generate CAN messages")
-	inputDir := command.
-		Arg("input-dir", "input directory").
+	fileOrDir := command.
+		Arg("input", "input DBC file, if <input> is a directory all contained DBC files will be generated").
 		Required().
-		ExistingDir()
+		ExistingFileOrDir()
 	outputDir := command.
 		Arg("output-dir", "output directory").
 		Required().
 		String()
 	command.Action(func(_ *kingpin.ParseContext) error {
-		return filepath.Walk(*inputDir, func(p string, i os.FileInfo, err error) error {
+		return filepath.Walk(*fileOrDir, func(p string, i os.FileInfo, err error) error {
 			if err != nil {
 				return err
 			}
 			if i.IsDir() || filepath.Ext(p) != ".dbc" {
 				return nil
 			}
-			relPath, err := filepath.Rel(*inputDir, p)
+			relPath, err := filepath.Rel(*fileOrDir, p)
 			if err != nil {
 				return err
 			}
+			if relPath == "." {
+				// happens if fileOrDir points directly to the DBC file
+				relPath = filepath.Base(*fileOrDir)
+			}
 			outputFile := relPath + ".go"
 			outputPath := filepath.Join(*outputDir, outputFile)
 			return genGo(p, outputPath)