content: use PageType in _unit_header.tmpl and unit_details.tmpl

Instead of computing the page type using .Unit methods, use the already
computed page type in these templates.

Change-Id: I7b724b7ca64027aa6a2defbc6d806f6afaa1e095
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/261719
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

content/static/html/helpers/_unit_header.tmpl

@@ -6,11 +6,7 @@
 
 {{define "unit_header"}}
   <header class="UnitHeader" role="complementary"
-      aria-label="{{if .Unit.IsCommand}}command
-      {{else if .Unit.IsPackage}}package
-      {{else if .Unit.IsModule}}module
-      {{else}}directory{{end}}
-      {{.Title}} information">
+      aria-label="{{if eq .PageType "std"}}module{{else}}{{.PageType}}{{end}} {{.Title}} information">
     <div class="UnitHeader-container">
       <div class="UnitHeader-breadcrumb">
         {{with .Breadcrumb}}

content/static/html/pages/unit_details.tmpl

@@ -12,11 +12,8 @@
   <div class="UnitDetails">
     {{.MobileOutline}}
     <div class="UnitDetails-outline" role="navigation"
-        aria-label="{{if .Unit.IsCommand}}command
-        {{else if .Unit.IsPackage}}package
-        {{else if .Unit.IsModule}}module
-        {{else}}directory{{end}}
-        details navigation">
+        aria-label="{{if eq .PageType "std"}}module
+        {{else}}{{.PageType}}{{end}}details navigation">
       {{block "unit_outline" .}}{{end}}
     </div>
     <div class="UnitDetails-content" role="main">