Add WebPackageResolver that uses a predefined URL to query package list

[Oleksii Holub]

Initialized with a URL, which is queried, expecting a plain-text response like this one:

1.0 https://some.website.org/files/package-1.0.zip
1.1 https://some.website.org/files/package-1.1.zip
2.0 https://other.resource.com/files/package-2.0.zip

This allows developers to use their own web server for providing updates.