go/packages: don't crash if given an invalid overlay

If the user of go/packages passes an overlay package that results in a
package having Go files in multiple directories, return an error instead
of crashing.

This is more of a bandaid than a fix for anything, but go/packages is
hard to follow even with full data. Without it I don't really have a
clue, so this is the best I've got.

Fixes golang/go#43520, I guess.

Change-Id: I37537ae2e0126f715719273ed9a6708ed53b850f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/282732
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

Author: heschi

go/packages/golist_overlay.go

@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"go/parser"
 	"go/token"
-	"log"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -35,9 +34,12 @@ func (state *golistState) processGolistOverlay(response *responseDeduper) (modif
 		// This is an approximation of import path to id. This can be
 		// wrong for tests, vendored packages, and a number of other cases.
 		havePkgs[pkg.PkgPath] = pkg.ID
-		x := commonDir(pkg.GoFiles)
-		if x != "" {
-			pkgOfDir[x] = append(pkgOfDir[x], pkg)
+		dir, err := commonDir(pkg.GoFiles)
+		if err != nil {
+			return nil, nil, err
+		}
+		if dir != "" {
+			pkgOfDir[dir] = append(pkgOfDir[dir], pkg)
 		}
 	}
 
@@ -441,20 +443,21 @@ func extractPackageName(filename string, contents []byte) (string, bool) {
 	return f.Name.Name, true
 }
 
-func commonDir(a []string) string {
+// commonDir returns the directory that all files are in, "" if files is empty,
+// or an error if they aren't in the same directory.
+func commonDir(files []string) (string, error) {
 	seen := make(map[string]bool)
-	x := append([]string{}, a...)
-	for _, f := range x {
+	for _, f := range files {
 		seen[filepath.Dir(f)] = true
 	}
 	if len(seen) > 1 {
-		log.Fatalf("commonDir saw %v for %v", seen, x)
+		return "", fmt.Errorf("files (%v) are in more than one directory: %v", files, seen)
 	}
 	for k := range seen {
-		// len(seen) == 1
-		return k
+		// seen has only one element; return it.
+		return k, nil
 	}
-	return "" // no files
+	return "", nil // no files
 }
 
 // It is possible that the files in the disk directory dir have a different package