Add `replace` field for wpackagist

[strarsis]

Summary

The WP Packages wp-plugin packages should declare that they replace their wpackagist-plugin counterparts, e.g. for Polylang:

"replace": {
    "wpackagist-plugin/polylang": "self.version"
}

The reason is that some plugins explicitly require a wpackagist-plugin package, e.g. polylang-slug:

{
	"name"        : "grappler/polylang-slug",
	"description" : "Allows same slug for multiple languages in Polylang",
	"keywords"    : [ "wordpress", "polylang" ],
	"type"        : "wordpress-plugin",
	"homepage"    : "https://github.com/grappler/polylang-slug",
	"license"     : "GPL-2.0-or-later",
	"authors"     : [
		{
			"name"    : "Ulrich Pogson",
			"homepage": "https://ulrich.pogson.ch/"
		}
	],
	"support"     : {
		"issues": "https://github.com/grappler/polylang-slug/issues"
	},
	"require" : {
		"php"                        : ">=5.3",
		"wpackagist-plugin/polylang" : "*"
	}
}

wpackagist-plugin/polylang forces the wpackagist-plugin package to be required.