internal/walk: add follow directives to follow symlinks (#338)

'# gazelle:follow link' directives can now be written in build
files. Gazelle will symbolic links to directories named in these
directives. Gazelle normally doesn't follow symbolic links within a
repository.

Care must be taken to avoid visiting a directory more than once. The
'# gazelle:exclude' directive may be used for this.
4 files changed