Print relative paths in crystal tool unreachable

[straight-shoota]

This patch relativizes paths in the output of crystal tool unreachable using Path#relative_to. The result is a more concise display without losing information.

Before:

/home/johannes/src/crystal-lang/crystal/foobar.cr:1:1     top-level foo   3 lines
/home/johannes/src/crystal-lang/crystal/foobar.cr:7:1     top-level foo   3 lines
/home/johannes/src/crystal-lang/crystal/foobar.cr:13:1    top-level bar   3 lines

After:

foobar.cr:1:1     top-level foo   3 lines
foobar.cr:7:1     top-level foo   3 lines
foobar.cr:13:1    top-level bar   3 lines

crystal tool dependencies uses the same mechanic already.