Replace "%s" with %q; octal literals for file permissions

[alexandear]

This PR:

• replaces "%s" with %q https://pkg.go.dev/fmt#hdr-Printing;
• replaces 0644 with 0o644 https://go.dev/ref/spec#Integer_literals ;
• removes unused function join;
• adds the test for StringToImportsOrders.

[codecov[bot]]

Codecov Report

Merging #121 (28ab41a) into master (365f9b6) will increase coverage by 0.80%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   88.72%   89.53%   +0.80%     
==========================================
  Files           8        8              
  Lines         621      621              
==========================================
+ Hits          551      556       +5     
+ Misses         49       45       -4     
+ Partials       21       20       -1     

Flag	Coverage Δ
unittests	89.53% <66.66%> (+0.80%)	:arrow_up:

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Files Changed	Coverage Δ
reviser/dir.go	64.70% <0.00%> (ø)
reviser/import_order.go	100.00% <100.00%> (+11.62%)	:arrow_up:

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