Use the `WPINC` constant when loading `class-phpass.php`

Props wojtek.szkutnik
See #14157.

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`WP_Filesystem_Base->abspath()` should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

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`wlwmanifest_link()` should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

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Admin screens should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

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`WP_Scripts->in_default_dir()` should use the `WPINC` constant

Props wojtek.szkutnik
See #14157.

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Use `includes_url()` in `wlwmanifest_link()`.

Props nacin.
See #14157.

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Convert documentation of default arguments in `wp_list_authors()` to the hash-notation style.

Props Viper007Bond.
Fixes #28684.

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Add initial unit tests for multisite's bootstrap.

props jeremyfelt.
fixes #27884.

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Use `includes_url( ..., 'relative' )` in `script-loader.php` in lieu of hard-coding `/wp-includes/....` everywhere.

Fixes #14157.

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