Migrate datatree.py module into xarray.core.

[owenlittlejohns]

This PR migrates the datatree.py module to xarray/core/datatree.py, as part of the on-going effort to merge xarray-datatree into xarray itself.

Most of the changes are import path changes, and type-hints, but there is one minor change to the methods available on the DataTree class: to_array has been converted to to_dataarray, to align with the method on Dataset. (See conversation here)

This PR was initially published as a draft here.

• [x] Completes migration step for datatree/datatree.py https://github.com/pydata/xarray/issues/8572
• [x] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

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[owenlittlejohns]

@TomNicholas - I think this PR is at a point now where it can be reviewed again in earnest. 🤞

[TomNicholas]

With @shoyer 's blessing (in the meeting) I hereby merge this PR

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