Use pkg-config to search for PETSc

Get rid of the cmake-modules package which is no more required.

packages/cmake-modules/.gitrepo

-; DO NOT EDIT (unless you know what you are doing)
-;
-; This subdirectory is a git "subrepo", and this file is maintained by the
-; git-subrepo command. See https://github.com/git-commands/git-subrepo#readme
-;
-[subrepo]
-	remote = https://github.com/jedbrown/cmake-modules.git
-	branch = master
-	commit = 91f96174a8b3f65e19519fa592b1571391c0e3d0
-	parent = 14615ef666ef3a26dcccd36f0a357b493e6cfbe5
-	method = merge
-	cmdver = 0.4.1