Fix generation of facts file when diagnostics are ignored (#4422)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

After #4402, it is no longer possible to use a custom analyzer without
having to declare a new `Fact` type. The build of a Go library will fail
if the library contains any external dependencies.

That's the case for executing gazelle by example (if we don't use a
precompiled binary):

```
$ bazel run //:gazelle
ERROR: .../external/gazelle++go_deps+org_golang_x_sync/errgroup/BUILD.bazel:3:11: output 'external/gazelle++go_deps+org_golang_x_sync/errgroup/errgroup.facts' was not created
ERROR: .../external/gazelle++go_deps+org_golang_x_sync/errgroup/BUILD.bazel:3:11: Running nogo on @@gazelle++go_deps+org_golang_x_sync//errgroup:errgroup failed: not all outputs were created or valid
ERROR: .../external/gazelle++go_deps+com_github_bazelbuild_buildtools/labels/BUILD.bazel:3:11: output 'external/gazelle++go_deps+com_github_bazelbuild_buildtools/labels/labels.facts' was not created
ERROR: .../external/gazelle++go_deps+com_github_bazelbuild_buildtools/labels/BUILD.bazel:3:11: Running nogo on @@gazelle++go_deps+com_github_bazelbuild_buildtools//labels:labels failed: not all outputs were created or valid
ERROR: .../external/gazelle++go_deps+in_gopkg_yaml_v3/BUILD.bazel:5:11: output 'external/gazelle++go_deps+in_gopkg_yaml_v3/yaml_v3.facts' was not created
ERROR: .../external/gazelle++go_deps+in_gopkg_yaml_v3/BUILD.bazel:5:11: Running nogo on @@gazelle++go_deps+in_gopkg_yaml_v3//:yaml_v3 failed: not all outputs were created or valid
ERROR: .../external/gazelle++go_deps+org_golang_x_sys/execabs/BUILD.bazel:3:11: output 'external/gazelle++go_deps+org_golang_x_sys/execabs/execabs.facts' was not created
ERROR: .../external/gazelle++go_deps+org_golang_x_sys/execabs/BUILD.bazel:3:11: Running nogo on @@gazelle++go_deps+org_golang_x_sys//execabs:execabs failed: not all outputs were created or valid
...
```

**Other notes for review**

This review continue to skip analysers when ignoring diagnostics, but
return a valid `pkg` to create the missing `.facts` file required by the
validation action, and incidentally, fix a potential crash at line
https://github.com/bazel-contrib/rules_go/blob/1f993522f463c142c5516f94852c3d6597f717c6/go/tools/builders/nogo_main.go#L98.

Author: lbcjbb

go/tools/builders/nogo_main.go

@@ -93,12 +93,26 @@ func run(args []string) (error, int) {
 	if err != nil {
 		return fmt.Errorf("error running analyzers: %v", err), nogoError
 	}
+
 	// Write the facts file for downstream consumers before failing due to diagnostics.
 	if *xPath != "" {
-		if err := os.WriteFile(abs(*xPath), pkg.facts.Encode(), 0o666); err != nil {
+		var factsContent []byte
+		if pkg != nil {
+			factsContent = pkg.facts.Encode()
+		}
+
+		if err := os.WriteFile(abs(*xPath), factsContent, 0o666); err != nil {
 			return fmt.Errorf("error writing facts: %v", err), nogoError
 		}
 	}
+
+	var fset *token.FileSet
+	if pkg != nil {
+		fset = pkg.fset
+	} else if len(diagnostics) > 0 {
+		return fmt.Errorf("pkg should not be nil with diagnostics"), nogoError
+	}
+
 	exitCode := nogoSuccess
 	var errMsg bytes.Buffer
 	if len(diagnostics) > 0 {
@@ -110,11 +124,11 @@ func run(args []string) (error, int) {
 		}
 		errMsg.WriteString("errors found by nogo during build-time code analysis:")
 		for _, d := range diagnostics {
-			fmt.Fprintf(&errMsg, "\n%s: %s (%s)", pkg.fset.Position(d.Pos), d.Message, d.analyzerName)
+			fmt.Fprintf(&errMsg, "\n%s: %s (%s)", fset.Position(d.Pos), d.Message, d.analyzerName)
 		}
 	}
 
-	if errs := saveSuggestedFixes(*nogoFixDir, diagnostics, pkg); len(errs) > 0 {
+	if errs := saveSuggestedFixes(*nogoFixDir, diagnostics, fset); len(errs) > 0 {
 		errMsg.WriteString("\nsaving suggested fixes:")
 		for _, err := range errs {
 			fmt.Fprintf(&errMsg, "\n%v", err)
@@ -127,12 +141,12 @@ func run(args []string) (error, int) {
 	return nil, exitCode
 }
 
-func saveSuggestedFixes(nogoFixDir string, diagnostics []diagnosticEntry, pkg *goPackage) []error {
+func saveSuggestedFixes(nogoFixDir string, diagnostics []diagnosticEntry, fset *token.FileSet) []error {
 	if nogoFixDir == "" {
 		return nil
 	}
 	var errs []error
-	fixes, err := getFixes(diagnostics, pkg.fset)
+	fixes, err := getFixes(diagnostics, fset)
 	if err != nil {
 		errs = append(errs, err)
 	}