FindPostgreSQL: Fix regression in computation of library directory

Since commit 368bcba2 (FindPostgreSQL: Find debug lib, imported
configs, 2019-05-09, v3.15.0-rc1~93^2) the `PostgreSQL_LIBRARY` variable
may contain a list and therefore should not be passed to the
`get_filename_component` command directly.  Instead spell out the logic
to compute `PostgreSQL_LIBRARY_DIR` from one of the per-configuration
variables.

Fixes: #19444
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