Second attempt to solve #5795.
This PR updates #6004 and replace it.

The dependency tree is read from the library_index.json, downloaded from arduino.cc, this json is build from the information contained in the library.properties of each library so, to complete this PR, we should add a field in the library.properties to explicitly specify dependencies.
In this PR the dependent libraries are handled without versions constraints, this will be added in a future development.

The latest commit has been added for testing purposes and it will be removed before merging this PR: it disables the automatic download of library_index.json and reads the index from library_index_test.json, this way we can test this PR by providing a fake library_index_test.json.

This is the index that I used for testing: library_index_test.zip that contains a modified fake entry:

    {
      "name": "ArduinoCloud",
      "version": "1.0.0",
      "author": "Arduino",
      "maintainer": "Arduino \u003cinfo@arduino.cc\u003e",
      "sentence": "Easly connect your Arduino/Genuino board to the Arduino Cloud",
      "paragraph": "Easly connect your Arduino/Genuino board to the Arduino Cloud",
      "website": "https://github.com/arduino-libraries/ArduinoCloud",
      "category": "Communication",
      "architectures": [
        "*"
      ],
      "types": [
        "Arduino"
      ],
      "requires": [
        { "name": "WiFi101" },
        { "name": "ArduinoSound" }
      ],
      "url": "http://downloads.arduino.cc/libraries/github.com/arduino-libraries/ArduinoCloud-1.0.0.zip",
      "archiveFileName": "ArduinoCloud-1.0.0.zip",
      "size": 45097,
      "checksum": "SHA-256:679a73db1aa4825652882ff71d7a3fb4992f4f79b41cb4a0a6c5c912adbfa848"
    },

and this is the dialog that appears when we try to install "ArduinoCloud":

(of course ArduinoCloud doesn't need ArduinoSound, this is just for testing multiple dependencies)

This PR requires a change in the library indexer (to report requires field in library_index.json) that is begin done right now.