Bump to Ignition v2.0.0-alpha
    
    Go back to git master for ignition-dracut and import Ignition from git
    tag v2.0.0-alpha. This required adapting the build to using go modules
    by building with `-mod=vendor` and `GO111MODULE=on` and dropping the
    `GOPATH` hacks.
    
    We also drop `parsedeps.go` since Ignition moved away from glide.
    Instead introduce a new tiny `go-mods-to-bundled-provides.py` script
    that just parses `go.mod` and `vendor/modules.txt` to generate virtual
    `Provides`.  The `parsedeps.go` approach also had a few issues that are
    fixed in the new script. Notably, it would only pick up on subpackages
    of dependent repos, and not the main packages (e.g. it skipped over
    `ajeddeloh/go-json`). And also, it was generating `Provides` at the
    version of the Ignition package (using `%{version}`), instead of the
    version of the bundled package itself.