acme_add_public_headers added, untested
start implementing acme_install

- replace ACME_FIND_PACKAGE_TARGETS with ACME_FIND_PACKAGE_SCOPED_TARGETS
- establish ACME_INCLUDE_DIR, ACME_PACKAGE_INCLUDE_DIR variables
- add acme_install_public_headers_from_target_add_public_headers
- add acme_install_public_headers_marked_public
- modify acme_init to allow setting slashed/dotted package include dir