This is the first phase of refactoring the internal structure of the npm
commands to set us up for future changes. This iteration changes the
function signature of `exec` for all the commands to be a async (no more
callbacks), and also groups all the commands into their own
subdirectory.
It also removes the Proxy `npm.commands` object, in favor of an
`npm.cmd` and `npm.exec` function that breaks up the two things that
proxy was doing. Namely, getting to the attributes of a given command
(`npm.cmd` now does this), and actually running the command `npm.exec`
does this.
PR-URL: https://github.com/npm/cli/pull/3959
Credit: @wraithgar
Close: #3959
Reviewed-by: @lukekarrys
