Bug 1429457 - [mozlint] Create formal 'setup' mechanism for bootstrapping lint dependencies, r=gbrown

This allows linters to define a 'setup' method which will automatically be
called by |mach lint| before running the linter. Users can also explicitly run
these methods (without doing any actual linting) by running |mach lint --setup|.

MozReview-Commit-ID: 74aY1pfsaX1

tools/lint/python/flake8.py

@@ -136,14 +136,14 @@ def run_process(config, cmd):
         proc.kill()
 
 
-def lint(paths, config, **lintargs):
-
+def setup(root):
     if not reinstall_flake8():
         print(FLAKE8_INSTALL_ERROR)
         return 1
 
-    binary = get_flake8_binary()
 
+def lint(paths, config, **lintargs):
+    binary = get_flake8_binary()
     cmdargs = [
         binary,
         '--format', '{"path":"%(path)s","lineno":%(row)s,'