refactor: rename and split Match_search_rules.ml in many files

[aryx]

This file was getting big

test plan:
make

PR checklist:

• Documentation is up-to-date
• Changelog is up-to-date
• Change has no security implications (otherwise, ping security team)

[linear]

PA-1448 metavariable-pattern not flagging with PHP code

Describe the bug
Metavariable-pattern is flagging PHP code with 'generic' mode, but not with 'php' mode.

To Reproduce
For instance, this doesn't flag: https://semgrep.dev/s/WRD2

But this does: https://semgrep.dev/s/02Wj

Expected behavior
Would expect it to flag with php as the language.

Priority
How important is this to you?

• P0: blocking me from making progress
• P1: this will block me in the near future
• P2: annoying but not blocking me

From #5443
Originally opened by colleend