Currently line linters (linters that open a file and process it line by line,
by applying a regex for example), don't handle directories. If a directory is
passed in, it will try to 'open' it, which fails. Directories can get hit if
the linter has a directory in its include directive or if the user passes in
--no-filter.
This patch modifies LineLinters so that if a directory is detected, we search
for all relevant files under that directory. If 'extensions' is used, we'll
look for only files with appropriate extensions. Otherwise we assume the
linter wants every file.
MozReview-Commit-ID: D9lzTNuQTob
2e5770fc84