# Your code using an array of resources for the require attribute.
file { "/etc/pki/tls/private/${::hostname}.example.com.key":
  source  => "puppet:///modules/bacula/${::hostname}/${::hostname}.example.com.key",
  owner   => 'bacula',
  group   => 'bacula',
  mode    => '0400',
  require => [
    Package['bacula-client','bacula-common'],
    File['/etc/pki/tls/private'],
    File['/etc/pki/tls/certs'],
  ],
  notify  => Service['bacula-fd'],
}

# Same resource without the already implicit resource requirement
file { "/etc/pki/tls/private/${::hostname}.example.com.key":
  source  => "puppet:///modules/bacula/${::hostname}/${::hostname}.example.com.key",
  owner   => 'bacula',
  group   => 'bacula',
  mode    => '0400',
  require => [
    Package['bacula-client','bacula-common'],
    File['/etc/pki/tls/certs'],
  ],
  notify  => Service['bacula-fd'],
}