[POC] refactor: hide internal modules

[wzhudev]

这个 PR 的意义是测试我们能否隐藏一些内部实现的模块，避免被用户所感知。

在实现二级入口的时候留下了一个问题：

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重新导出二级入口的内容

在确保二级入口能够正确打包之后，我们还必须确保一级入口能够重新导出（re-export）所有二级入口的内容，以保持对旧的引入方式的兼容。方法是修改 ng-zorro-antd.module.ts 中对组件的导出方式：

+ export * from 'ng-zorro-antd/affix';
- export * from './affix/';

然而 ng-packagr 的一个 bug 会造成导出同名变量的冲突。原因是，ngc 要求导出一个 module 下所有的 directive component service 和子 module，如果开发者没有在入口文件中显式的导出它们，ngc 就会隐式地导出，例如 date picker 组件的入口文件 ng-zorro-antd-date-picker.d.ts 里就有一些隐式导出的 component 和 module：

export * from './public-api';
export { AbstractPickerComponent as ɵp } from './abstract-picker.component';
export { DateRangePickerComponent as ɵo } from './date-range-picker.component';
export { HeaderPickerComponent as ɵr } from './header-picker.component';
export { CalendarFooterComponent as ɵd } from './lib/calendar/calendar-footer.component';
export { CalendarHeaderComponent as ɵb } from './lib/calendar/calendar-header.component';
export { CalendarInputComponent as ɵc } from './lib/calendar/calendar-input.component';
export { OkButtonComponent as ɵe } from './lib/calendar/ok-button.component';
export { TimePickerButtonComponent as ɵf } from './lib/calendar/time-picker-button.component';
export { TodayButtonComponent as ɵg } from './lib/calendar/today-button.component';
export { DateTableComponent as ɵh } from './lib/date/date-table.component';
export { DecadePanelComponent as ɵl } from './lib/decade/decade-panel.component';
export { LibPackerModule as ɵa } from './lib/lib-packer.module';
export { MonthPanelComponent as ɵj } from './lib/month/month-panel.component';
export { MonthTableComponent as ɵk } from './lib/month/month-table.component';
export { DateRangePopupComponent as ɵn } from './lib/popups/date-range-popup.component';
export { InnerPopupComponent as ɵm } from './lib/popups/inner-popup.component';
export { YearPanelComponent as ɵi } from './lib/year/year-panel.component';
export { NzPickerComponent as ɵq } from './picker.component';

这在没有二级入口的情况下不会触发 bug，但是如果我们在一级入口重新导出，而且其他 module 也有这种隐式导出的话，就会发生导出重名变量的冲突。解决的办法是显式地导出这些变量。然而这带来了另外一个问题，即一些内部实现的东西也暴露给用户了。

比较合理解决方案是 ng-packagr 给 SEP 起一个有两级结构的 alias，比如 alias as θda 而不是 as θa。我们会在以后尝试修复掉这个瑕疵。

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原文在这里。

这个 PR 旨在修复该瑕疵，向用户隐藏内部，特别是较为复杂组件的细节。

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This pull request is to hide some implementation details from users, especially those of complicated components.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #4249 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4249      +/-   ##
==========================================
- Coverage   92.31%   92.31%   -0.01%     
==========================================
  Files         519      519              
  Lines       10963    10963              
  Branches     1984     1984              
==========================================
- Hits        10121    10120       -1     
  Misses        419      419              
- Partials      423      424       +1

Impacted Files	Coverage Δ
components/time-picker/time-holder.ts	95.61% <0%> (-0.88%)	⬇️

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[wzhudev]

这里只是一个实验，大家可以在修改自己的组件时决定是否需要隐藏内部实现。