Handle include statements

[adamconnelly]

Thrift has the ability to include and reference items from other thrift files. For example:

include "UserType.thrift"

struct User {
    1: UserType.UserType Type,
    2: Username Username
}

• Error if the file doesn't exist.
• Warn if the file is unused.
• Add the file to the list of files to compile if it isn't already included based on the list of input files to the command.
• Resolve any symbols referenced from the include.
• Error if any references can't be found.