resources: Create a common ResourceFinder interface

And make both .Resources and resources implement it.

This gets us 2 new methods/functions, so you can now also do:

* .Resources.Get
* resources.ByType

Note that GetRemote is not covered by this interface, as that is only available as a global template function.

Fixes #8653

hugolib/hugo_sites_build_test.go

@@ -397,6 +397,8 @@ func doTestMultiSitesBuild(t *testing.T, configTemplate, configSuffix string) {
 	c.Assert(bundleFr, qt.Not(qt.IsNil))
 	c.Assert(len(bundleFr.Resources()), qt.Equals, 1)
 	logoFr := bundleFr.Resources().GetMatch("logo*")
+	logoFrGet := bundleFr.Resources().Get("logo.png")
+	c.Assert(logoFrGet, qt.Equals, logoFr)
 	c.Assert(logoFr, qt.Not(qt.IsNil))
 	b.AssertFileContent("public/fr/bundles/b1/index.html", "Resources: image/png: /blog/fr/bundles/b1/logo.png")
 	b.AssertFileContent("public/fr/bundles/b1/logo.png", "PNG Data")