[gogenproto] Allow specifying a Go package mapping for import paths (#36

)

### Background

→ It may be the case that you're importing protos that are generated in
some other way than with `gogenproto` but which similarly have a
conventional Go path mapping (e.g. they don't specify `go_package` but
instead get automatic package names. Unfortunately right now that means
you can't depend on these protos from your `gogenproto`-using protos,
since you can't tell `gogenproto` what their package name will be.

### Changes

- Add an optional Go package prefix mapping to import paths. By
appending `=` and the Go package prefix, you can use conventional
(directory based) path mapping.

### Testing

- Tested in stately monorepo

gogenproto/README.md

@@ -59,21 +59,17 @@ package models
 → ./path/to/bin/gogenproto --help
 Usage of ./bin/gogenproto:
   -grpc
-        also generate grpc service definitions (experimental)
+    	also generate grpc service definitions (experimental)
   -include value
-        comma-separated paths to additional packages to include
+    	comma-separated paths to additional directories to add to the proto include path. You can set an optional Go package mapping by appending a = and the package path, e.g. foo=github.com/foo/bar
   -input-dir string
-        path to root directory for proto generation (env PWD)
+    	path to root directory for proto generation (env PWD)
   -inputDir string
-        path to root directory for proto generation (env PWD)
-  -output-dir string
-        relative output path for generated files (default "../")
-  -outputDir string
-        relative output path for generated files (default "../")
+    	path to root directory for proto generation (env PWD)
   -recurse
-        generate protos recursively
+    	generate protos recursively
   -vt-proto
-        also generate vtproto
+    	also generate vtproto
 ```
 
 ### TODO:

gogenproto/gen/generate.go

@@ -22,7 +22,7 @@ type Generate struct {
 	Recurse bool     `default:"false" usage:"generate protos recursively"`
 	VTProto bool     `default:"false" usage:"also generate vtproto"`
 	GRPC    bool     `default:"false" usage:"also generate grpc service definitions (experimental)"`
-	Include []string `usage:"comma-separated paths to additional packages to include"`
+	Include []string `usage:"comma-separated paths to additional directories to add to the proto include path. You can set an optional Go package mapping by appending a = and the package path, e.g. foo=github.com/foo/bar"`
 
 	// TODO: add flags for other languages, TS, etc.
 	// TODO: add NATIVE validation support.
@@ -52,7 +52,8 @@ func (g Generate) Run() error {
 		)
 	}
 	includePaths := append([]string{g.InputDir}, g.Include...)
-	for _, path := range includePaths {
+	for _, pathAndMaybePkg := range includePaths {
+		path, pkgPrefix, hasPkgPrefix := strings.Cut(pathAndMaybePkg, "=")
 		includePath, err := filepath.Abs(path)
 		if err != nil {
 			return err
@@ -63,14 +64,23 @@ func (g Generate) Run() error {
 			return err
 		}
 		for _, path := range protoImportPaths {
-			pkg, err := gencommon.PackageNameFromPath(filepath.Dir(path))
-			if err != nil {
-				return err
-			}
 			relPath, err := filepath.Rel(includePath, path)
 			if err != nil {
 				return err
 			}
+
+			var pkg string
+			if hasPkgPrefix {
+				// Use the explicit package mapping
+				pkg = filepath.Join(pkgPrefix, filepath.Dir(relPath))
+			} else {
+				// Auto detect package name from directory
+				pkg, err = gencommon.PackageNameFromPath(filepath.Dir(path))
+				if err != nil {
+					return err
+				}
+			}
+
 			mapping := relPath + "=" + pkg
 
 			args = append(args,