Don't lint file content of directories

Summary: This also changes how we treat @generated and @nolint - after this diff, we will still lint their path.

Test Plan:
Created directory a.php and symlink b.php pointing to directory.
arc lint previously printed:

Requested path `/data/users/jakubv/devtools/arcanist/_.php' is not a file.

Now it prints:

No lint warnings.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3737