Use get_site_by() in get_id_from_blogname()

[jeremyfelt]

The introduction of get_site_by() (see #40180) gives us the opportunity to clean up some similar code.

[jeremyfelt]

In 41743:

Multisite: Use get_site_by() in get_id_from_blogname().

Fixes #42091.

[flixos90]

As get_site_by() will be reverted (see https://core.trac.wordpress.org/ticket/40180#comment:36), this needs to be reverted as well. It can be merged back in once get_site_by() is actually ready.

[flixos90]

In 41884:

Multisite: Revert [41698] and [41743].

In order for get_site_by() to be truly beneficial, caching in WP_Site_Query needs to be improved to account for common use-cases and have them be invalidated less aggressively.

See #40180, #40228, #42091.

[jeremyfelt]

get_site_by() isn't a function until #40180 happens, so this can be closed as maybelater.