Golang library for validating package against specs

[ycombinator]

This PR creates a go module that provides an API for validating a package against the appropriate specification version.

Usage

import "github.com/elastic/package-spec/code/go/pkg/validator"

validationErrs := validator.ValidateFromPath("/path/to/package/root/folder")

where validationErrs will implement the error interface type.

Resolves #4.

Author TODO

• Implement folder structure validation
• Implement file content validation

[ycombinator]

I decided to break up this PR into smaller PRs for easier review. So I'm closing it in favor of #12, #14, and a future PR to implement file content validation.