Stage Go sources in a directory named after the package.

This is a workaround for the issue described at
cockroachdb/cockroach#64379 -- this is
unnecessarily inefficient, but it does work and avoids some regressions
`rules_go` would otherwise introduce.

go/tools/builders/cgo2.go

@@ -195,14 +195,19 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 
 	// Copy regular Go source files into the work directory so that we can
 	// use -trimpath=workDir.
-	goBases, err := gatherSrcs(workDir, goSrcs)
+	subDir := filepath.Join(workDir, packagePath)
+	err = os.MkdirAll(subDir, 0755)
+	if err != nil {
+		return "", nil, nil, err
+	}
+	goBases, err := gatherSrcs(subDir, goSrcs)
 	if err != nil {
 		return "", nil, nil, err
 	}
 
 	allGoSrcs = make([]string, len(goSrcs)+len(genGoSrcs))
 	for i := range goSrcs {
-		allGoSrcs[i] = filepath.Join(workDir, goBases[i])
+		allGoSrcs[i] = filepath.Join(subDir, goBases[i])
 	}
 	copy(allGoSrcs[len(goSrcs):], genGoSrcs)
 	return workDir, allGoSrcs, cObjs, nil

go/tools/builders/compilepkg.go

@@ -284,7 +284,16 @@ func compileArchive(
 				return err
 			}
 		}
-		gcFlags = append(gcFlags, "-trimpath=.")
+		subDir := filepath.Join(workDir, packagePath)
+		err := os.MkdirAll(subDir, 0755)
+		if err != nil {
+			return err
+		}
+		goBases, err := gatherSrcs(subDir, goSrcs)
+		for i, base := range goBases {
+			goSrcs[i] = filepath.Join(subDir, base)
+		}
+		gcFlags = append(gcFlags, "-trimpath="+workDir)
 	}
 
 	// Check that the filtered sources don't import anything outside of