Declare the sitemaps of every shop in robots.txt on multistore

[ShaiMagal]

Questions	Answers
Description?	Declares the sitemap of every shop in robots.txt on multistore, instead of only the one in context. Details below.
Type?	bug fix
BC breaks?	no
Deprecations?	no
Fixed ticket?	Fixes #206
How to test?	See below
Sponsor company	OpenServis.cz

Why

robots.txt is a single file in the shop root and multistore serves it to every domain, but the core writes a Sitemap line only for the shop in context (Tools::generateRobotsFile()). The sitemaps of the remaining shops are then declared nowhere, and those shops are served a Sitemap line for a domain that is not theirs.

What it does

Subscribes to actionAdminMetaAfterWriteRobotsFile, which the core fires with the open file handle, and appends one Sitemap line per remaining shop using that shop's own domain and its own PS_SSL_ENABLED value. The shop in context is skipped because the core already wrote it, and only shops whose sitemap file exists and is non-empty are listed, same condition the core applies. It does nothing when multistore is off.

The hook has to reach existing installations, hence the version bump and upgrade-5.1.0.php.

How to test

1. Multistore on, two shops on separate domains
2. Generate the sitemap for both shops in the module
3. Shop Parameters > Traffic & SEO > Generate robots.txt file
4. Fetch robots.txt

Before:

# Sitemap
Sitemap: https://shop-one.example/1_index_sitemap.xml

After:

# Sitemap
Sitemap: https://shop-one.example/1_index_sitemap.xml
Sitemap: https://shop-two.example/2_index_sitemap.xml

Multiple Sitemap lines are allowed, so each domain now finds its own. Worth checking that a single-shop install produces a byte-identical file to before, and that a shop with no generated sitemap is not listed.