Use ensuredir() instead of os.makedirs() to fix race conditions

Use the ensuredir() function consistently across Sphinx code to avoid
race conditions e.g. when multiple Sphinx instances attempt to create
the same output directory. The ensuredir() function that was already
present in sphinx.util.osutil correctly catches EEXIST exception that
occurs if the specified directory already exists (i.e. it was created
between the call to os.path.isdir() and os.makedirs() that follows it).

While at it, remove redundant os.path.isdir() calls when they only
guarded the os.makedirs() call, and replace mkdir_p() which had pretty
much the same purpose, except for being prone to race conditions.

I did not modify testing-related code as race conditions mostly affect
real applications and not the test environment.

Fix #4281: Race conditions when creating output directory

CHANGES

@@ -33,6 +33,7 @@ Bugs fixed
 * #4279: Sphinx crashes with pickling error when run with multiple processes and
   remote image
 * #1421: Respect the quiet flag in sphinx-quickstart
+* #4281: Race conditions when creating output directory
 
 Testing
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