Implements "yarn policies set-version"

[arcanis]

Summary

This feature implements a new command:

$> yarn policies set-version [range]
# Examples:
$> yarn policies set-version nightly
$> yarn policies set-version rc
$> yarn policies set-version ^1.12
$> yarn policies set-version 1.12.3

When run, Yarn will download from Github the latest release matching the specified range, will put it inside the <lockfileFolder>/.yarn/releases directory, and will update <lockfileFolder>/.yarnrc to point to this new binary.

Essentially, this command allows:

• to upgrade Yarn in a single swoop
• to rely on npm much less (which always was a bit funny)
• to enforce a specific Yarn version for a single project

This last point is especially important: we've been recommending people to use the same Yarn release across a team, but they usually had to discover by themselves that the yarn-path settings existed (despite it being one of the selling points in term of build reproducibility). Once they did they loved it, but it wasn't an easy find.

Test plan

I'm not too sure how to test this automatically, but I've tested it manually and it works just fine.

[buildsize]

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	1.1 MB	1.1 MB	43 bytes (0%)
yarn-[version].js	4.46 MB	4.47 MB	5.25 KB (0%)
yarn-legacy-[version].js	4.65 MB	4.66 MB	5.46 KB (0%)
yarn-v[version].tar.gz	1.11 MB	1.12 MB	3.01 KB (0%)
yarn_[version]all.deb	813.62 KB	814.53 KB	928 bytes (0%)

[arcanis]

I'm considering renaming this command to yarn policies set-version in order to implement other such "auto-configure" policies in the future (for example yarn policies inline-cache to automatically configure the cache to be local to the project, etc).