Fix: README etc. are only assumed published if in pkg root (fixes #73)

[platinumazure]

This implements one of the approaches I outlined in issue #73 to ensure that files like test/readme.js (has a basename equal to something that npm auto-publishes, but is not in the package root) are not treated as published and so can require devDependencies.

Specifically, when using the NEVER_IGNORED regex, this PR will pass the full relative path from package root to the linted file, instead of the linted file's basename. All existing tests pass with this logic, and my new tests also pass (but failed with the old logic).

Please let me know if I should make any changes.

[codecov-io]

Codecov Report

Merging #74 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #74   +/-   ##
======================================
  Coverage    98.7%   98.7%           
======================================
  Files          38      38           
  Lines        1001    1001           
======================================
  Hits          988     988           
  Misses         13      13

Impacted Files	Coverage Δ
tests/lib/rules/no-unpublished-import.js	100% <ø> (ø)	⬆️
lib/util/get-npmignore.js	100% <ø> (ø)	⬆️
tests/lib/rules/no-unpublished-require.js	100% <ø> (ø)	⬆️

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