Remove all instances of extern crate. Not needed.

[scouten]

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment. n/a

[codecov]

Codecov Report

Merging #72 into main will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          655       655           
=========================================
  Hits           655       655           

Impacted Files	Coverage Δ
src/object/check_tree.rs	100.00% <ø> (ø)
src/object/file_content_source.rs	100.00% <ø> (ø)
src/object/id.rs	100.00% <ø> (ø)
src/object/mod.rs	100.00% <ø> (ø)
src/path/mod.rs	100.00% <ø> (ø)
src/test_support/temp_git_repo.rs	100.00% <ø> (ø)

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