fix: Import issues with hyphen named files

[butler54]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commits are signed-off.

Import had an issue, due to using some legacy parsing functionality which was broken, which meant that importing a 'system-security-plan' or other file with a hyphenated name would fail.

This PR corrects the issue - and cleans up some of the code in the parser and import to decrease but not eliminate duplication of code.

[codecov]

Codecov Report

Merging #371 (5d452c4) into develop (a5510aa) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #371      +/-   ##
===========================================
- Coverage    99.12%   99.12%   -0.01%     
===========================================
  Files           53       54       +1     
  Lines         6489     6481       -8     
===========================================
- Hits          6432     6424       -8     
  Misses          57       57              

Impacted Files	Coverage Δ
trestle/core/commands/import_.py	100.00% <100.00%> (ø)
trestle/core/parser.py	94.64% <100.00%> (-0.82%)	⬇️
trestle/oscal/component.py	100.00% <100.00%> (ø)
trestle/__init__.py	100.00% <0.00%> (ø)

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[fsuits]

LGTM. All tests passed on my windows machine.