cmd/go/internal/load: prevent calling ImportErrorf when the err is *module.InvalidPathError

1. The call of setError is unnecessary bacause setError do nothing when
p.Error is not nil.
2. The call of ImportErrorf may panic in this situation
(double quotes appear in the import path).

Fixes golang#49137

Author: SilverRainZ

src/cmd/go/internal/load/pkg.go

@@ -1844,12 +1844,18 @@ func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *
 	if other := foldPath[fold]; other == "" {
 		foldPath[fold] = p.ImportPath
 	} else if other != p.ImportPath {
-		setError(ImportErrorf(p.ImportPath, "case-insensitive import collision: %q and %q", p.ImportPath, other))
+		var invalidPathErr *module.InvalidPathError
+		if !errors.As(err, &invalidPathErr) {
+			setError(ImportErrorf(p.ImportPath, "case-insensitive import collision: %q and %q", p.ImportPath, other))
+		}
 		return
 	}
 
 	if !SafeArg(p.ImportPath) {
-		setError(ImportErrorf(p.ImportPath, "invalid import path %q", p.ImportPath))
+		var invalidPathErr *module.InvalidPathError
+		if !errors.As(err, &invalidPathErr) {
+			setError(ImportErrorf(p.ImportPath, "invalid import path %q", p.ImportPath))
+		}
 		return
 	}