[beta.55] private fields transform produces unnecessary WeakMaps, and not-private output.

## Bug Report

I played around with the new private fields [in the repl](https://babeljs.io/repl/build/master/#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=MYGwhgzhAEBiD29oG8BQ1oGIBmjoF5oBWdLAIzACcDoA2UgFwFMIGAKAShVI2HgDsI8EEwB0IeAHM2DABYBLCKJyIANNDmLlFSh1IBfVIdR9BDaNhr8mAdziJU2Uc1acgA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2&prettier=false&targets=&version=7.0.0-beta.55&envVersion=) and discovered some things:

- The fields aren't truly private. The output code can be imported by script tag (for example) and it leaks the private fields into public space.
- They are only "private" strictly to the Babel output code.
- See this [codepen example](https://codepen.io/trusktr/project/editor/82a736a7537498166dac1ad31a5a554c) of public code accessing private fields.
- The `_foo` and `_bar` variables shouldn't be accessible. Babel should expose only the variables explicitly defined in the original source.
- Unnecessary number of WeakMaps are created, one for each field.
- If the fields aren't truly private, there's no need to create `WeakMaps` at all.
- We don't know where users will stick output code (f.e. a `` tag, not necessarily a closure), so output should be truly-private, otherwise the performance hit of making `WeakMap`s is entirely defeated.
- In many cases people might use Node `require` or `Webpack`, which automatically wraps code in a `function` thus creating encapsulation, but this isn't always the case.

See my truly-private implementation idea: https://github.com/babel/proposals/issues/12#issuecomment-410115104

Please feel free to borrow any ideas from there.

Start with the linked Babel REPL example and CodePen, reproduce the beta.55 private-fields transform, and inspect the generated output. Read the linked proposals discussion for the intended privacy semantics. Done means the output does not expose private names and avoids unnecessary per-field WeakMaps.

Written by the indexing model from the issue text.