Add checks package for runtime condition verification

[d3sch41n]

Summary: This feature will power verification of:

• TTP Prerequisites (right tools being installed, running in correct environment, etc)
• Verification that individual steps return the correct results

I'm adding it as an isolated package for now, in a subsequent diff I will hook it into step.go like this:

// CommonStepFields contains the fields
// common to every type of step (such as Name).
// It centralizes validation to simplify the code
type CommonStepFields struct {
    Name        string         `yaml:"name,omitempty"`
    Description string         `yaml:"description,omitempty"`
    Checks      []checks.Check `yaml:"checks,omitempty"`

Note that this initial diff only contains one type of check (path existence) but others will be added shortly - wanted to keep it small to get comments on the overall structure first.

Differential Revision: D51428503

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51428503

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51428503

[d3sch41n]

Orphan PR - already merged by ShipIt