Make PathConversion package private.
Add test for liberal path to key conversion.

The new filesystem PathConversion can also understand data layouts that
do not include the field name in partition directories. This commit
updates the FileSystemPartitionIterator to keep paths unchanged so it
also supports malformed directory names.

TestFileSystemPartitionIteratorTolerance is a new test that verifies the
iterator works normally when directories are mixed.
CDK-208: Update DatasetDescriptor#format(String).

The format(String) method now delegates to Formats.fromString to look up
the format by name.