put pg_catalog implicitly on the search_path when it isn't there already

[zachmu]

pg_catalog is always implicitly the first element on the search_path, unless it's explicitly included elsewhere:

https://www.postgresql.org/docs/current/ddl-schemas.html#DDL-SCHEMAS-CATALOG

We weren't implementing this behavior, causing us to fail to resolve ::regclass expressions in some cases with a "no such relation" error for all pg_catalog tables.

This fix isn't complete, because the same logic should also apply to all table lookups. i.e. users should be able to create a table named pg_class, and what the name pg_class resolves to depends on whether the pg_catalog schema is explicitly present in the search_path and where it is relative to the user's other schemas. That's a pretty obscure bug and is left unaddressed for now.

Fixes #2217

[github-actions]

	Main	PR
covering_index_scan_postgres	536.43/s	526.43/s	-1.9%
index_join_postgres	147.29/s	149.45/s	+1.4%
index_join_scan_postgres	260.06/s	262.62/s	+0.9%
index_scan_postgres	13.21/s	13.26/s	+0.3%
oltp_point_select	2622.02/s	2635.91/s	+0.5%
oltp_read_only	2010.38/s	2094.89/s	+4.2%
select_random_points	137.77/s	139.47/s	+1.2%
select_random_ranges	421.60/s	413.07/s	-2.1%
table_scan_postgres	12.93/s	13.00/s	+0.5%
types_table_scan_postgres	6.03/s	6.01/s	-0.4%

[github-actions]

	Main	PR
Total	42090	42090
Successful	17563	17564
Failures	24527	24526
Partial Successes1	5582	5582

	Main	PR
Successful	41.7273%	41.7296%
Failures	58.2727%	58.2704%

${\color{lightgreen}Progressions (1)}$

regproc

QUERY: SELECT to_regclass('pg_class');

Footnotes

1. These are tests that we're marking as Successful, however they do not match the expected output in some way. This is due to small differences, such as different wording on the error messages, or the column names being incorrect while the data itself is correct. ↩

[Hydrocharged]

LGTM! There are several places that don't use the search path function since they manually handle (or try to handle) the pg_catalog part, but those can be cleaned up in a different PR.