Sitemaps: rename 'sitemap' to 'provider' in registry class

[swissspidy]

This would have helped prevent a bug like #50666.

This class is for registering sitemap providers, not individual sitemaps.

[SergeyBiryukov]

In 48536:

Sitemaps: Rename 'sitemap' to 'provider' in WP_Sitemaps_Registry class.

This class is for registering sitemap providers, not individual sitemaps.

Props swissspidy.
Fixes #50724.

[pbiron]

50724.2.diff​ additionally changes:

1. wp_get_sitemaps() to wp_get_sitemaps_providers()
2. a couple of occurrences of $sitemaps = $registry->get_providers() to $providers = $registry->get_providers() in the unit tests

[swissspidy]

In 48540:

Sitemaps: Rename wp_get_sitemaps() to wp_get_sitemaps_providers()

Following [48536], rename the function to match the rest of the sitemaps logic.

Also eliminates some dead code after [48523].

Props pbiron.
See #50724. See #50643.

[SergeyBiryukov]

In 48541:

Sitemaps: Rename wp_get_sitemaps_providers() to wp_get_sitemap_providers().

Follow-up to [48536], [48540].

Props pbiron, swissspidy, SergeyBiryukov.
See #50724.

[pbiron]

rename wp_register_sitemap() to wp_register_sitemap_provider()

[pbiron]

Reviewing the draft sitemaps dev-note, I noticed one more function that needs to be renamed: wp_register_sitemap() to wp_register_sitemap_provider().

50724.4.diff​ does that.

[SergeyBiryukov]

In 48553:

Sitemaps: Rename wp_register_sitemap() to wp_register_sitemap_provider().

Follow-up to [48536], [48540], [48541].

Props pbiron.
Fixes #50724.