Add ‘sources’ option to ::getPropertyValue

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Fix ::get memoization, except when options are provided

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Use key-path-helpers instead of underscore-plus for key path functions
Handle global selectors in ::mergeMatchingPropertySets