Library manager: dependency resolver (take 2)

[cmaglie]

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 \u003c[email protected]\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.

[sandeepmistry]

This is looking good!

However, the modal window appeared on the top left hand side, when testing this on macOS 10.14.4.

[cleveHEX]

@cmaglie
How this works in CLI? If I run arduino --install-library "DHT sensor library", will it install Adafruit Unified Sensor (I mean if config was set properly)?

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8600-BUILD-887-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8600-BUILD-887-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8600-BUILD-887-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8600-BUILD-887-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-8600-BUILD-887-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.