cmake: find scotch with cmake config first

[minrk]

• setting explicit SCOTCH_DIR still has highest priority for user override
• next, try finding scotch with cmake config (will usually work with scotch >=7, I think)
• if scotch cmake config is found and defines the target SCOTCH::ptscotch, populate scotch libraries/includes from cmake targets
• otherwise, proceed as before with FindSCOTCH

Intended effect:

• no change if SCOTCH_DIR is specified
• no change if scotch cmake config is not present

WIth this, scotch 7 installed via conda is found correctly by default via cmake at least on my mac and Windows tests

[garth-wells]

3244 should achieve some thing, but is simpler.

[garth-wells]

Closing in favour of #3244