layouts: correctly check for template existence

https://github.com/jpanther/congo/commit/4e4b470915171a5855f3f2ea25851995aace8b55
fixed the theme for building with Hugo v0.146.0 or later but it
doesn't correctly check for template existence. That is, if you set
`homepage.layout` to `profile` this theme will still load the `page`
layout.

Hugo's [templates.Exists](https://gohugo.io/functions/templates/exists/)
function tests for the existence of templates relative to the `layouts`
directory. The example code they provide uses `printf` to join the
template name with the `_partials` directory, which would be `partials`
instead for this theme.

```
{{ $partialPath := printf "headers/%s.html" .Type }}
{{ if templates.Exists ( printf "_partials/%s" $partialPath ) }}
  {{ partial $partialPath . }}
{{ else }}
  {{ partial "headers/default.html" . }}
{{ end }}
```

layouts/_default/baseof.html

@@ -21,7 +21,7 @@
       >
     </div>
     {{ $header := print "header/" .Site.Params.header.layout ".html" }}
-    {{ if templates.Exists $header }}
+    {{ if templates.Exists ( printf "partials/%s" $header) }}
       {{ partial $header . }}
     {{ else }}
       {{ partial "header/basic.html" . }}

layouts/index.html

@@ -1,6 +1,6 @@
 {{ define "main" }}
   {{ $partial := print "home/" .Site.Params.homepage.layout ".html" }}
-  {{ if templates.Exists $partial }}
+  {{ if templates.Exists ( printf "partials/%s" $partial) }}
     {{ partial $partial . }}
   {{ else }}
     {{ partial "home/page.html" . }}