Fix ExpandPath so it always returns a clean path and resolves "~" to the home directory

[anuragkumar19]

Issues

1. Currently, ExpandPath does a filepath.IsAbs check at the start, and if the path is absolute, it returns the path unmodified. This skips the cleaning normally performed by filepath.Join or filepath.Abs on other code paths. For example, the path /a/c/../c is returned unmodified.
2. The path ~ doesn't resolve to the home directory; instead, it gets joined with the working dir by filepath.Abs.

Fixes

1. Remove the filepath.IsAbs check. If the path is already absolute, filepath.Abs will clean and return it.
2. Add handling to resolve ~ to the home directory.
3. Add tests for ExpandPath.

Tests

• go test ./... passes.