Create Index(Output) class and migrate index generation from Site

Like previous Content refactor, this allows validating file collisions
in the discover/write build pipeline.

omicron/ssg/templates/plain/index.html

@@ -1,6 +1,6 @@
 {% extends "base.html" %}
-{% set prefix = (section + "/") if section else (("tags/" + tag + "/") if tag else "") %}
-{% set heading = section or tag or "" %}
+{% set prefix = (label + "/") if kind == "section" else (("tags/" + label + "/") if kind == "tag" else "") %}
+{% set heading = label or "" %}
 
 {% block title %}{% if heading %}{{ heading }} — {% endif %}{{ site.name }}{% if page_num > 1 %} — Page {{ page_num }}{% endif %}{% endblock %}
 
@@ -10,16 +10,16 @@
 {% endblock %}
 
 {% block breadcrumb %}
-{% if section %}
+{% if kind == "section" %}
 <nav aria-label="breadcrumb">
     <a href="{{ site.base_dir }}">{{ site.name }}</a> ›
-    {{ section }}
+    {{ label }}
 </nav>
-{% elif tag %}
+{% elif kind == "tag" %}
 <nav aria-label="breadcrumb">
     <a href="{{ site.base_dir }}">{{ site.name }}</a> ›
     <a href="{{ site.base_dir }}tags/">Tags</a> ›
-    {{ tag }}
+    {{ label }}
 </nav>
 {% else %}
 {{ super() }}
@@ -40,11 +40,11 @@
 {% if total_pages > 1 %}
 <nav>
     {% if page_num > 1 %}
-    <a href="{{ site.base_dir }}{{ prefix }}page-{{ page_num - 1 }}.html">← Newer</a>
+    <a href="{{ site.base_dir }}{{ prefix }}{{ "" if page_num - 1 == 1 else "page-" ~ (page_num - 1) ~ ".html" }}">← Newer</a>
     {% endif %}
     {% for p in range(1, total_pages + 1) %}
     {% if p == page_num %}<strong>{{ p }}</strong>
-    {% else %}<a href="{{ site.base_dir }}{{ prefix }}page-{{ p }}.html">{{ p }}</a>
+    {% else %}<a href="{{ site.base_dir }}{{ prefix }}{{ "" if p == 1 else "page-" ~ p ~ ".html" }}">{{ p }}</a>
     {% endif %}
     {% endfor %}
     {% if page_num < total_pages %}