generator: add validation of subprojects

specifically:
- only sigs+committees can have subprojects
- subprojects need at least one owners entry
- owners should be raw github links

I think the last one could/should change, but:
- I'm about to auto-generate links assuming this format, so I want to
  verify I can assume all owners are in this format
- I am not quite sure if we should treat sigs.yaml as public api and
  give notice/deprecation for field changes

generator/app.go

@@ -246,6 +246,7 @@ func (c *Context) Sort() {
 func (c *Context) Validate() []error {
 	errors := []error{}
 	people := make(map[string]Person)
+	rawGitHubURL := regexp.MustCompile(regexRawGitHubURL)
 	for prefix, groups := range c.PrefixToGroupMap() {
 		for _, group := range groups {
 			expectedDir := group.DirName(prefix)
@@ -294,6 +295,21 @@ func (c *Context) Validate() []error {
 					errors = append(errors, fmt.Errorf("%s: has no subprojects", group.Dir))
 				}
 			}
+			if prefix != "committee" && prefix != "sig" {
+				if len(group.Subprojects) > 0 {
+					errors = append(errors, fmt.Errorf("%s: only sigs and committees can own code / have subprojects, found: %v", group.Dir, group.Subprojects))
+				}
+			}
+			for _, subproject := range group.Subprojects {
+				if len(subproject.Owners) == 0 {
+					errors = append(errors, fmt.Errorf("%s/%s: subproject has no owners", group.Dir, subproject.Name))
+				}
+				for _, ownerURL := range subproject.Owners {
+					if !rawGitHubURL.MatchString(ownerURL) {
+						errors = append(errors, fmt.Errorf("%s/%s: subproject owners should match regexp %s, found: %s", group.Dir, subproject.Name, regexRawGitHubURL, ownerURL))
+					}
+				}
+			}
 		}
 	}
 	return errors

generator/testdata/sigs.yaml

@@ -6,6 +6,8 @@ sigs:
     mission_statement: covers foo
     subprojects:
     - name: sub-foo
+      owners:
+      - "https://raw.githubusercontent.com/org/foo/main/OWNERS"
   - dir: sig-bar
     name: Bar
     label: bar
@@ -13,6 +15,8 @@ sigs:
     mission_statement: owns areas related to bar
     subprojects:
     - name: sub-bar
+      owners:
+      - "https://raw.githubusercontent.com/org/bar/main/test/OWNERS"
 workinggroups:
   - dir: wg-baz
     name: Baz