Arduino boards and libraries should be installable without internet

In various situations, ranging from restrictive or expensive venues, to remote areas with poor internet service, it is desirable to be able to install the Arduino IDE (multiple compilers, and libraries) without a viable internet connection.

Ideally, I think you should be able to read the .json files from a directory, and have the content picked from the same place. Perhaps as a preference: "install from: [Internet | "
(not unlike searching for new windows drivers, in fact.)

### Additional context

#### Additional requests

- https://github.com/arduino/Arduino/issues/2981#issuecomment-93497749
- https://github.com/arduino/Arduino/issues/2981#issuecomment-312645453

Start by reviewing how the Arduino IDE currently obtains board definitions, compilers, and libraries, then read the referenced additional-context comments. Define an offline installation flow that reads the relevant .json files and content from a user-selected directory. Done means boards, libraries, and compilers can be installed without an internet connection.

Written by the indexing model from the issue text.