Don't load `esmify` for projects that don't use UMD

[talentlessguy]

esmify depends on a deprecated core-js module and also on babel-runtime. And it's only loaded if UMD is set as target output. I'd suggest marking esmify as an optional dependency, it's dynamically loaded already anyway.

@gqlts/cli 3.2.19-beta.88
└─┬ esmify 2.1.1
└─┬ babel-plugin-import-to-require 1.0.0
└─┬ babel-template 6.26.0
├─┬ babel-runtime 6.26.0
│ └── core-js 2.6.12
├─┬ babel-traverse 6.26.0
│ ├─┬ babel-messages 6.23.0
│ │ └─┬ babel-runtime 6.26.0
│ │   └── core-js 2.6.12
│ ├─┬ babel-runtime 6.26.0
│ │ └── core-js 2.6.12
│ └─┬ babel-types 6.26.0
│   └─┬ babel-runtime 6.26.0
│     └── core-js 2.6.12
└─┬ babel-types 6.26.0
└─┬ babel-runtime 6.26.0
└── core-js