* New yaml document type !require to map IIDs to IIDs that required them.
* !require file to be read before copy and updated after copy.
* New variables:
VENDOR_DIR_NAME - name of the local folder to hold the index and require files
SITE_BOOKKEEPING_DIR path to local bookkeeping dir e.g. /Library/Applications
SITE_VENDOR_BOOKKEEPING_DIR e.g. /Library/Applications/Waves
SITE_REPO_BOOKKEEPING_DIR e.g. /Library/Applications/Waves/V9

SITE_REQUIRE_FILE_PATH: $(SITE_REPO_BOOKKEEPING_DIR)/require.yaml
NEW_SITE_REQUIRE_FILE_PATH: $(SITE_REQUIRE_FILE_PATH).new

* InstallItem.requirement_of renamed required_by