Aerc usually used the path []int{1} if it didn't know what the proper path is.
However this only works for multipart messages and breaks if it isn't one.
This patch removes all the hard coding and extracts the necessary helpers to lib.
When message dates failed to parse, the error displayed would try to
include the time object it failed to obtain, which would display as
something like 0001-01-01 00:00:00 UTC, which isn't of much help.
Instead, display the text we were trying to parse into a date, which
makes the problem easier to debug.
Things like FetchEntityPartReader etc can be reused by most workers
working with raw email files from disk (or any reader for that matter).
This patch extract that common functionality in a separate package.
