refactor json modules

[vankop]

What kind of change does this PR introduce?
refactoring. fixes #15789

• do not store exports spec
• return undefined exports spec for large arrays
• add memoryLimitCases test cases, this cases allow to test heap size using testConfig.heapSizeLimitBytes

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[sokra]

Thanks