Relax mandatory need for relative base paths

[ruffsl]

by using os.path.relpath() instead of path.relative_to(), allowing for absolute paths that are not sub paths to CWD.

Fixes https://github.com/colcon/colcon-clean/issues/37

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c7ff24c) 100.00% compared to head (f35cef8) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #41 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 268 269 +1 Branches 40 41 +1 ========================================= + Hits 268 269 +1 ``` | [Files](https://app.codecov.io/gh/colcon/colcon-clean/pull/41?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=colcon) | Coverage Δ | | |---|---|---| | [colcon\_clean/subverb/\_\_init\_\_.py](https://app.codecov.io/gh/colcon/colcon-clean/pull/41?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=colcon#diff-Y29sY29uX2NsZWFuL3N1YnZlcmIvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | |

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[Benjamin-Tan]

@cottsay do you have any comments on this PR?