suite init и suite add: заведение набора и конвенции в двух режимах

- suite init создаёт директорию и манифест со скелетом таблиц; suite add пишет
  файл конвенции и вправляет запись в suite.toml, сохраняя комментарии
- без аргументов команды спрашивают поля с подсказками, с флагами берут всё
  сразу и не спрашивают ничего; без терминала пустой вызов отказывает
- строка о версии языка генерируется из словаря набора, поэтому созданный файл
  проходит suite check без правок
- починено разрешение extends: короткая форма бралась по суффиксу и могла
  указать на сам файл; теперь неоднозначность либо избегается при записи, либо
  сообщается ошибкой

internal/check/spread.go

@@ -3,6 +3,7 @@ package check
 import (
 	"path"
 	"regexp"
+	"sort"
 	"strings"
 
 	"git.vakhrushev.me/av/convy/internal/doc"
@@ -74,7 +75,13 @@ func checkExtends(s *suite.Suite, d *doc.Document, rep *Report) {
 		return
 	}
 	at := d.Front.At["extends"]
-	target := resolveExtends(s, d.Front.Extends)
+	target, others := resolveExtends(s, d, d.Front.Extends)
+	if len(others) > 1 {
+		rep.Errorf(Spread, d.Path, at,
+			"extends points at %q, and the suite holds several files it could mean (%v): give the path from the root of the suite",
+			d.Front.Extends, others)
+		return
+	}
 	if target == nil {
 		rep.Errorf(Spread, d.Path, at,
 			"extends points at %q, and the suite holds no such file", d.Front.Extends)
@@ -91,17 +98,52 @@ func checkExtends(s *suite.Suite, d *doc.Document, rep *Report) {
 	}
 }
 
-// resolveExtends looks up the document at the path written in extends. The path
-// is given from the conventions directory, so both it and a path from the root
-// of the suite are tried.
-func resolveExtends(s *suite.Suite, ref string) *doc.Document {
+// resolveExtends looks up the document at the path written in extends.
+//
+// The path may be given from the root of the suite or from the directory of
+// conventions, so an exact match is tried first and a tail match second. A tail
+// match can fit several files at once — two layers of one topic often share a
+// file name — so every candidate is returned and the caller reports the
+// ambiguity instead of picking by the order documents happen to be loaded in.
+// The document doing the extending is never its own base.
+func resolveExtends(s *suite.Suite, from *doc.Document, ref string) (*doc.Document, []string) {
 	ref = path.Clean(strings.TrimPrefix(ref, "./"))
+	var candidates []*doc.Document
 	for _, d := range s.Docs {
-		if d.Path == ref || strings.HasSuffix(d.Path, "/"+ref) {
-			return d
+		if d == from {
+			continue
+		}
+		if d.Path == ref {
+			return d, []string{d.Path}
+		}
+		if strings.HasSuffix(d.Path, "/"+ref) {
+			candidates = append(candidates, d)
 		}
 	}
-	return nil
+	paths := make([]string, len(candidates))
+	for i, d := range candidates {
+		paths[i] = d.Path
+	}
+	sort.Strings(paths)
+	if len(candidates) == 1 {
+		return candidates[0], paths
+	}
+	return nil, paths
+}
+
+// namesSuiteFile reports whether a candidate written in the text names a file
+// the suite holds. Unlike an extends key it needs no single answer: a path that
+// fits several files of the canon is a path all the same.
+func namesSuiteFile(s *suite.Suite, candidate string) bool {
+	if s.Exists(candidate) {
+		return true
+	}
+	for _, d := range s.Docs {
+		if d.Path == candidate || strings.HasSuffix(d.Path, "/"+candidate) {
+			return true
+		}
+	}
+	return false
 }
 
 // checkMechanized looks for the mark of mechanization in the text of a
@@ -142,8 +184,7 @@ func checkCanonPaths(s *suite.Suite, d *doc.Document, rep *Report) {
 			continue
 		}
 		for _, candidate := range mdPathRe.FindAllString(d.Line(n), -1) {
-			target := resolveExtends(s, candidate)
-			if target == nil && !s.Exists(candidate) {
+			if !namesSuiteFile(s, candidate) {
 				continue
 			}
 			rep.Errorf(Spread, d.Path, n,