extract kindsys to external library (#64562)

* extract kindsys

* reinstate kindsys report

This may end up living somewhere else (or not! who knows!), but the important part is that I don't get rid of it right now :)

I hate the package layout (kindsysreport/codegen) for the main function and will take pretty much any alternative suggestion, but we can change also change it later.

Note that the generated report.json is in a different location - anything using this (ops something) needs to be updated.

* kindsysreport in codeowners

pkg/kindsysreport/codeowners.go

@@ -0,0 +1,61 @@
+package kindsysreport
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/hmarr/codeowners"
+)
+
+type CodeOwnersFinder struct {
+	ruleset codeowners.Ruleset
+}
+
+func NewCodeOwnersFinder(groot string) (CodeOwnersFinder, error) {
+	//nolint:gosec
+	file, err := os.Open(filepath.Join(groot, ".github", "CODEOWNERS"))
+	if err != nil {
+		return CodeOwnersFinder{}, err
+	}
+
+	ruleset, err := codeowners.ParseFile(file)
+	if err != nil {
+		return CodeOwnersFinder{}, err
+	}
+
+	return CodeOwnersFinder{
+		ruleset: ruleset,
+	}, nil
+}
+
+func (f CodeOwnersFinder) FindFor(pp ...string) ([]string, error) {
+	if len(f.ruleset) == 0 {
+		return nil, nil
+	}
+
+	// Set, to avoid duplicates
+	m := make(map[string]struct{})
+
+	for _, p := range pp {
+		r, err := f.ruleset.Match(p)
+		if err != nil {
+			return nil, err
+		}
+
+		// No rule found for path p
+		if r == nil {
+			continue
+		}
+
+		for _, o := range r.Owners {
+			m[o.Value] = struct{}{}
+		}
+	}
+
+	result := make([]string, 0, len(m))
+	for k := range m {
+		result = append(result, k)
+	}
+
+	return result, nil
+}