cmPackageInfoReader: Fix handling of "definitions"

Rewrite cmPackageInfoReader's parsing of the "definitions" attribute.
The old logic (having been originally adapted from proof-of-concept
parsing code circa 2023) was parsing the attribute according to its
specification as of CPS 0.11, but the representation was changed in
CPS 0.12. Add a test to verify that definitions are being imported
correctly. Remove unnecessary setting and resetting of CMAKE_PREFIX_PATH
in the test.