internal/build: ignore some files in FindMainPackages

[jsvisa]

In macOS, there may exist hidden .DS_Store files inside some directories. These are filesystem metadata files created by Finder.
If one of these .DS_Store files is inside the ./cmd directory, then running make test will fail with an error:

$ make test
env GO111MODULE=on go run build/ci.go install  
util.go:202: fdopendir cmd/.DS_Store: not a directory  
exit status 1
make: *** [all] Error 1

This happens because the Go build tools try to process all files in the ./cmd directory, including the .DS_Store file, resulting in an error.

To fix this, we can ignore any files when parsing the package directory.

[holiman]

Makes sense to me