cmd/go/internal/vet: skip -fix on pkgs from vendor or non-main mod

This change causes go fix (and go vet -fix) to skip applying fixes
to any package in the vendor/ tree, including the GOROOT vendor
packages that are part of std, and to any package from a non-main
module (since these usually come from the readonly module cache).

+ test

Fixes golang/go#76479

Change-Id: Ifdb73e09fbe413b4d99a92e5081b8ea43460be0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/727300
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

src/cmd/go/internal/vet/vet.go

@@ -262,6 +262,15 @@ func run(ctx context.Context, cmd *base.Command, args []string) {
 	// will only be executed in VetxOnly mode, for facts but not
 	// diagnostics.
 	for _, p := range pkgs {
+		// Don't apply fixes to vendored packages, including
+		// the GOROOT vendor packages that are part of std,
+		// or to packages from non-main modules (#76479).
+		if applyFixes {
+			if p.Standard && strings.HasPrefix(p.ImportPath, "vendor/") ||
+				p.Module != nil && !p.Module.Main {
+				continue
+			}
+		}
 		_, ptest, pxtest, perr := load.TestPackagesFor(moduleLoaderState, ctx, pkgOpts, p, nil)
 		if perr != nil {
 			base.Errorf("%v", perr.Error)

src/cmd/go/testdata/script/fix_vendor.txt

@@ -0,0 +1,44 @@
+# Test that go fix skips fixes to non-main and/or vendored packages.
+# (It uses the interface{} -> any modernizer.)
+
+# Create vendor tree programmatically to avoid
+# having to hardcode sums in this txtar archive.
+go mod vendor
+
+# Show fixes on two packages, one in the main module
+# and one in a vendored dependency.
+# Only the main one (a) is shown.
+go fix -diff example.com/a example.com/b
+stdout 'a[/\\]a.go'
+stdout '\-var _ interface\{\}'
+stdout '\+var _ any'
+! stdout 'b[/\\]b.go'
+
+# Apply fixes to the same two packages.
+# Only the main module was modified.
+go fix example.com/a example.com/b
+grep 'var _ any'          a/a.go
+grep 'var _ interface{}'  b/b.go
+grep 'var _ interface{}'  vendor/example.com/b/b.go
+
+-- go.mod --
+module example.com
+go 1.26
+
+require "example.com/b" v0.0.0
+replace "example.com/b" => ./b
+
+-- a/a.go --
+package a
+
+import _ "example.com/b"
+
+var _ interface{}
+
+-- b/go.mod --
+module example.com/b
+
+-- b/b.go --
+package b
+
+var _ interface{}