This adds the boilerplate for selecting configurable tokenizers.
A tokenizer can be chosen at import time and will then install
itself such that it is fixed for the given database import even
when the software itself is updated.
The legacy tokenizer implements Nominatim's traditional algorithms.
- the command is now --import-special-phrases
- the output is not an sql file anymore, data are directly imported to the database.
- the little part on the documentation (section data import) has been modified.
As we can't refer to the project root dir in the module path, the
module path may now also be a relative directory which is then
taken as being relative to the project root path.
Moves the checkModulePresence() function into the Setup class, so
that it can work on the computed absolute module path.
