Move logic to package:icu4x

[mosuem]

Merge after unicode-org/icu4x#6739

Remove the build and link hook portions which are now in package:icu4x - this means that the code copying can be deleted as well.

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
intl4x	Breaking	0.12.2	0.13.0-wip	0.13.0	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
pkgs/intl4x/hook/link.dart	💔 Not covered
pkgs/intl4x/lib/src/case_mapping/case_mapping_4x.dart	💚 100 %
pkgs/intl4x/lib/src/collation/collation_4x.dart	💚 77 %
pkgs/intl4x/lib/src/datetime_format/datetime_format_4x.dart	💚 75 %
pkgs/intl4x/lib/src/display_names/display_names_4x.dart	💚 69 %
pkgs/intl4x/lib/src/list_format/list_format_4x.dart	💚 100 %
pkgs/intl4x/lib/src/locale/locale_4x.dart	💚 43 %
pkgs/intl4x/lib/src/number_format/number_format_4x.dart	💚 93 %
pkgs/intl4x/lib/src/plural_rules/plural_rules_4x.dart	💚 100 %

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

License Headers ✔️

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// BSD-style license that can be found in the LICENSE file.

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