BUG #18920: LOAD '$libdir/plugins' no longer works in 18beta1

The following bug has been logged on the website:

Bug reference:      18920
Logged by:          Evan Si
Email address:      evsi@amazon.com
PostgreSQL version: 18beta1
Operating system:   Amazon Linux 2
Description:

Hi,
The 18beta1 documentation for the LOAD command has remained the same since
previous major versions.
https://www.postgresql.org/docs/18/sql-load.html
In the postgres installation, I can copy something e.g. auto_explain into a
plugins subdirectory of `pg_config --pkglibdir`
```
cp $(pg_config --pkglibdir)/auto_explain.so $(pg_config
--pkglibdir)/plugins/auto_explain.so
```
then from the server start:
```
psql (18beta1)
Type "help" for help.
postgres=# LOAD '$libdir/plugins/auto_explain.so';
ERROR:  could not access file "plugins/auto_explain.so": No such file or
directory
postgres=# \q
```
This behavior differs from documentation and previous majors. It seems to me
that $libdir is getting stripped out from this commit:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=4f7f7b0
--
Evan