fix: exclude patterns ignored when scanning directories

[2-towns]

When passing a directory to nph, files found by walkDirRec were added to explicitFiles, causing exclude patterns to be bypassed entirely. Files in nimbledeps/ (and other default exclusions) were formatted when they shouldn't .

The fix defers directory walking to after patterns are compiled, applying exclusions to paths relative to the passed directory.

To reproduce:

mkdir -p /tmp/test_nph_exclude/nimbledeps                                                                                
echo 'let x=1' > /tmp/test_nph_exclude/main.nim                                                                        
echo 'let x=1' > /tmp/test_nph_exclude/nimbledeps/foo.nim

nph --check /tmp/test_nph_exclude products:

arnaud@fedora:/tmp/test_nph_exclude$ /home/arnaud/Work/logos/nph/nph --check .  
would reformat ./main.nim
would reformat ./nimbledeps/foo.nim

Oh no! 💥 💔 💥
2 files would be reformatted.

After the fix:

arnaud@fedora:/tmp/test_nph_exclude$ /home/arnaud/Work/logos/nph/nph --check .  
would reformat ./main.nim

Oh no! 💥 💔 💥
1 file would be reformatted.