Webpack view helpers that indicate pack type in the method name, e.g.
javascript_pack_tag, stylesheet_pack_tag, support manifest lookup both
with or without the extname in the given file name. For example, given
an existing pack for application.js, all of the following manifest
lookups will succeed:

```
javascript_pack_tag "application"    # or
javascript_pack_tag "application.js"

stylesheet_pack_tag "application"    # or
stylesheet_pack_tag "application.css"
```

When the lookup does not succeed, a helpful error message is displayed.
However, if the ".js" extension is omitted from the helper method
invocation, the error message also omits the pack type. For example:

```
javascript_pack_tag "not-a-pack"
stylesheet_pack_tag "not-a-pack"
```
results in the following error message:

```
Webpacker can't find not-a-pack in
/path/to/app/public/packs/manifest.json
```

The ambiguity is confusing when using `extract_css` true: does this
indicate the server was looking for the javascript bundle or the
stylesheet?

This changeset resolves the ambiguity by leveraging the existing support
in the manifest for determining the qualified pack name by passing type
to the method that handles the missing entry info.

Also, adds some assertions and extracts a couple shared methods in
manifest_test.rb