compilepkg: fix stored file path truncation

This fixes the issue described in cockroachdb/cockroach#64379 where
the internal paths of the files don't include the
`github.com/cockroachdb/cockroach` prefix.

We fix this by using the rewrite syntax of `-trimpath`, e.g.
`-trimpath=/sandbox/execroot=>github.com/cockroachdb/cockroach`.

Figuring out the replacement part is not trivial. We use the package
path and strip out the relative path of the source file directory.

go/tools/builders/compilepkg.go

@@ -365,14 +365,23 @@ func compileArchive(
 			goSrcsNogo = goSrcs
 		}
 
-		gcFlags = append(gcFlags, createTrimPath(gcFlags, srcDir))
+		gcFlags = append(gcFlags, fmt.Sprintf("-trimpath=%s=>%s", abs(srcDir), packagePath))
 	} else {
 		if cgoExportHPath != "" {
 			if err := ioutil.WriteFile(cgoExportHPath, nil, 0o666); err != nil {
 				return err
 			}
 		}
-		gcFlags = append(gcFlags, createTrimPath(gcFlags, "."))
+		// We want the source files to show up (e.g. in stack traces) with the package
+		// path. We use -trimpath to replace the root path with the correct prefix
+		// of the package path.
+		root := abs(".")
+		relSrcPath, err := filepath.Rel(root, srcs.goSrcs[0].filename)
+		if err != nil {
+			return err
+		}
+		rootPkgPath := filepath.Clean(strings.TrimSuffix(packagePath, filepath.Dir(relSrcPath)))
+		gcFlags = append(gcFlags, fmt.Sprintf("-trimpath=%s=>%s", root, rootPkgPath))
 	}
 
 	// Check that the filtered sources don't import anything outside of
@@ -560,7 +569,7 @@ func compileGo(goenv *env, srcs []string, packagePath, importcfgPath, embedcfgPa
 	args = append(args, "-linkobj", outLinkobjPath)
 	args = append(args, "--")
 	args = append(args, srcs...)
-	absArgs(args, []string{"-I", "-o", "-trimpath", "-importcfg"})
+	absArgs(args, []string{"-I", "-o", "-importcfg"})
 	return goenv.runCommand(args)
 }