Split out the attribute retrieval form auth-source-pass-get

Split out the attribute retrieval form auth-source-pass-get

Eliminate the need to repeatedly retrieve and parse the data for the
entry.  This is generally a good thing since it eliminates repetitions
of the same crypto and parsing operations.  It is especially valuable
when protecting an entry with a yubikey with touch required for crypto
operations as it eliminates the need to touch the yubikey sensor for
each attribute retrieved.

* lisp/auth-source-pass.el (auth-source-pass-get): Extract some code to
`auth-source-pass--get-attr'.
(auth-source-pass--get-attr): New function to get a field value from a
parsed entry.
(auth-source-pass--build-result): Make use of
`auth-source-pass--get-attr` to avoid repeated parsing.