This commit changes DependencyReason's list fields to maps and sets. Duplicate
flags were possible when a flag appeared multiple times in nested conditionals
or a flag controlled a "Buildable: False" field. The duplicate flag could show
up in log messages:

Before:
[__5] trying: json-rpc-client:+demo
[__6] trying: process-1.6.1.0/installed-1.6... (dependency of json-rpc-client +demo +demo)

After:
[__5] trying: json-rpc-client:+demo
[__6] trying: process-1.6.1.0/installed-1.6... (dependency of json-rpc-client +demo)