Decode Bowerfiles as lenient JSON and only parse necessary fields #211

In #210 we saw a number of packages fail with a MalformedBowerJson error, namely because of issues like this:

{
  "name": "purescript-facebook",
  "description": "Idiomatic Purescript bindings for the Facebook SDK",
  "license": "Apache-2.0",
  "keywords": "facebook purescipt",
  ...
}

This fails to decode as a Bower.PackageMeta because the keywords field has to be an array of keywords, not a plain string. We don't even use this field in our manifests, though, so we shouldn't care if it isn't Bower-compliant!

The errors aren't always just that the file can't be decoded as a Bower.PackageMeta. Sometimes they aren't valid JSON at all:

{
  "name": "purescript-endpoints-express",
  "version": "0.0.1",
  "authors": {
    "Simon Van Casteren <simon.van.casteren@gmail.com>"
  },
  ...
}

This file isn't valid JSON because of the authors key, which has an object with no key/value pairs as its value. We can't parse this as Json at all.

And yet we can pull off the keys that we know need to be there -- the name, the version, the dependencies, and the devDependencies -- and attempt to decode their contents. To avoid needlessly removing packages from the registry, I suggest that we change how we parse Bowerfiles once #210 merges so that packages like these can be converted into manifests despite their JSON issues.

There are some we maybe still can't parse as Json, because even the keys we're looking for have little issues. For example, the next one is even more insidious:

{
    "name": "purescript-uint",
    ...
    "dependencies": {
        "purescript-enums": "^v5.0.0",
        "purescript-gen": "^v3.0.0",
        "purescript-math": "^v3.0.0",
        "purescript-maybe": "^v5.0.0",
        "purescript-prelude": "^v5.0.0",
        "purescript-psci-support": "^v5.0.0",
    }
    "devDependencies": {
        "purescript-effect": "^v3.0.0",
        "purescript-quickcheck": "^v7.1.0",
        "purescript-quickcheck-laws": "^v6.0.1"
    }
}

There's a trailing comma after the "purescript-psci-support" dependency! It's not valid JSON! It would be nice if we could handle this case too, but perhaps we just pull off the keys we need and if they're not valid JSON then we just drop the package.