Fix filepath not correctly printed

[narumi147]

Problems:

• unexpected leading slash
• duplicated filename

Changes:

• use path lib to correctly handle relative path

Before:

contains flutter: true
contains registrant: false
┏━━ Sorting 586 dart files
┃  ┗━━ ✔ Sorted imports for /lib/subfolder/page.dart/page.dart
┗━━ ✔ Sorted 1 files in 0.932 seconds

After:

contains flutter: true
contains registrant: false
┏━━ Sorting 586 dart files
┃  ┗━━ ✔ Sorted imports for lib/subfolder/page.dart
┗━━ ✔ Sorted 1 files in 0.853 seconds
···

[narumi147]

@vkammerer Hello, can you have a check on this pr?

Btw, are you the only active maintainer now? I think it's better to add/update "active maintainers" on README and repo description (it still points to the original owner now)