When an hook is defined, a side effect was the creation of the
${notmuchIni.database.path}/.notmuch/ directory by home-manager. If
the Xapian database does not exist yet but this .notmuch directory
exists, Notmuch is confused and throws an error when `notmuch new` is
run (while this should create the database the first time).
This commit changes the hooks paths to $XDG_CONFIG_HOME where Notmuch
expects them (see notmuch-config(1)) instead of inside the maildir
database directory.
It also moves the configuration where Notmuch expects it, but the
$NOTMUCH_CONFIG environment variable is kept for backward
compatibility.
This reworks the way program specific email account options are
specified. In particular, we no longer use the deprecated `options`
field of `mkOption`. Instead submodules are used.