Global build vars

[rsora]

This PR aims to refactor a group of global variables in a global package in order to inject values at build time and use them in the CLI section of the cli

In order to have the values injected the use of -ldflags param is required in the go build phase otherwise a missing value will be shown (so an update in our CI is mandatory)

i.e. to inject the version value

go build -ldflags "-X github.com/arduino/arduino-cli/global.version=0.3.6-alpha.preview"

[CLAassistant]

All committers have signed the CLA.

[cmaglie]

I'm not sure about injecting values via ldflags, this is not very idiomatic.

If someone builds the cli from source with go build github.com/arduino/arudino-cli the version information is lost (well, on the other hand hardcoding the version gives only a rough approximation but at least you know from which version you're basing on).

Maybe we can keep the version hardcoded? it could be overriden by the CI anyway, so we get the best of the two approaches.

[rsora]

@cmaglie in accordance to your idea

keep the version hardcoded? it could be overriden by the CI anyway, so we get the best of the two approaches.

added hardcoded fallback version in 0408a77

[rsora]

PR #227 is built upon this PR with heavy refactoring of the content.

In agreement with the colleagues this PR will be closed