Add hacking check for explicit import of _

Unit tests are not guaranteed to discover whether
the _() function has been explicitly imported to a file.

To ensure that people do not add messages with _() translations
without importing the function, I am adding this hacking check
to enforce the import of _ when it is needed.

The commit includes fixes for offenders found by the new
check.

Change-Id: I616dce6a9da461b3ad2cfaf5e114a2aaa8ced9fa
Closes-Bug: #1348129