Move checkPath from DirCacheCheckout to ObjectChecker

The bulk of the "is this sane" logic is inside of ObjectChecker.  The
only caller for the version in DirCacheCheckout is an obtuse usage for
the static isValidRefName() method in Repository.

Deprecate the weird single use method in DirCacheCheckout and move all
code for checking a sequence of path components into ObjectChecker,
where it makes sense alongside the existing code that checks a single
component at a time.

Reuse a single ObjectChecker for the local platform, to avoid looking
up the system properties on each path string considered.

Change-Id: Iae6e769f2bfcad05c166e70ff255f9cf9fcdc87e