Switch datepicker to MDC

[crisbeto]

Switches the Material datepicker to be based on top of the MDC components.

[mmalerba]

LGTM

[mmalerba]

I realized that there's a potential issue with this. If we just switch it to the MDC version, people might not have the appropriate theme in their app.

We'll have to figure out how we want to handle this, here's a few ideas:

• Split out a legacy-datepicker that retains the legacy deps
• Have ng update automatically add the button theme when people update to v15
• Have the datepicker theme mixins @include the button one
• Have some back-compat mixin that ng update adds and ng generate removes after migrating the button:

  @mixin datepicker-back-compat-theme($theme) {
    .mat-datepicker {
      @include mat.button-theme($theme);
    }
  }
  

[crisbeto]

This is ready to review again. I've added a material/legacy-datepicker entrypoint with the minimum amount of code necessary to support it.

[crisbeto]

Closing in favor of #25648.

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