added go module for tools

[Code-Hex]

• added go.mod file in tools directory
• Currently, we need to download a lot of modules to resolve the dependencies for tools. Even though these modules are not used in go-fuzzyfinder package itself, but go-fuzzyfinder users are required to download them.

[codecov]

Codecov Report

Merging #129 (59209b5) into master (2123b55) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   86.96%   86.96%           
=======================================
  Files           5        5           
  Lines         752      752           
=======================================
  Hits          654      654           
  Misses         85       85           
  Partials       13       13           

[ktr0731]

It makes sense to reduce the main module dependencies.
Thanks for your Pull Request!