Import does not allow for database table prefixes

Problem/Motivation

when creating a Drupal database one of the options is to add a prefix to all tables (for example prefixing 'xxx' results in table name xxx_file_managed). If prefixing is used then, on running import, the error "SQLSTATE[42S02]: Base table or view not found: 1146 Table 'databasename.file_managed' doesn't exist: SELECT file_managed.uri FROM file_managed ORDER BY file_managed.fid DESC limit 1; Array
(
)" is raised

Steps to reproduce

1. create a taxonomy and add terms
2. export a csv of the taxonomy
3. import csv into different database without prefix on tables (no error)
4. repeat step 3 for database with prefix on table in step 3 (error occurs)

Proposed resolution

on the configuration page add a field for prefix, amend SQL to incorporate if used.

Remaining tasks

User interface changes

API changes

Data model changes