check file changed in loop

[tejal29]

small perf improvement.

1. check if a file is changed in gowalkdir which is faster than filepath.Walk.
2. Move file deletion logic in the same loop.

This PR reduces the number of times the whole filesystem is walked from 3 to just 1.

[tejal29]

Performance improvements

Number of Files | Total Build Time Full Snapshot - current | Total Build Time -  Walk performance | % decrease
-- | -- | -- | --
10000 | 13.834973 | 13.579324 | 1.85%
50000 | 47.926338 | 42.460665 | 11.40%
100000 | 96.440554 | 77.774629 | 19.35%
200000 | 187.346273 | 175.198485 | 6.48%
300000 | 341.233622 | 289.063343 | 15.29%
500000 | 1404.410121 | 1122.947834 | 20.04%
700000 | 1772.379601 | 1350.19682 | 23.82%