(PUP-3750) Support :include: in mailalias type

The mailalias type does not allow the specification of included files,
which contain an alias's elements. Such alias entries are commonly
specified in the an MTA's aliases file through the following syntax.

aliasname: :include: /path/to/file

Specifying the rest of an ":include:" line as a recipient does not
work, because the existing mailalias type will quote the entry.

This change adds a new parameter named "file" to the mailalias type,
which allows the indirect specification of a file containing the
recipients.  The parameter parses and generates ":include:" alias entries.
I don't think it makes a lot of sense to use `newproperty` but we use that in several other types that have parsedfile backed providers. As for whether the parsedfile provider will do the right thing, I think this could describe anything involving parsedfile - it might work but there are not guarantees. :)