ioutil.ReadDir returns []os.FileInfo, which contains information on
each entry in the directory that is only available by calling
os.Lstat on the entry. Finder only the name and type (regular,
directory or symlink) of the files, which on Linux kernels >= 2.6.4
is available in the return values of syscall.Getdents.
In preparation for using syscall.Getdents, switch filesystem.ReadDir
to return an interface that only contains the information that will
be available from syscall.Getdents.
Bug: 70897635
Test: m checkbuild
Change-Id: Id2749d709a0f7b5a61abedde68549d4bf208a568
