Previously, when ignoreUnknownFields was false and property names were
being filtered based on whether or not they begin with the target name,
relaxed variants of the target name were not considered. This resulted
in different delimiters resulting in a non-match. For example, the
property ENV_FOO_NAME would be filtered out when the target name
was env.foo.

This commit updates PropertiesConfigurationFactory to pass all of the
relaxed variants for the target name to the matcher. For the example
above one of those variants will be env_foo which matches ENV_FOO_NAME
due to the matching delimiter.

PropertiesConfigurationFactory was already creating a RelaxedNames
instance for the target name. The code has been reworked a little to
allow these relaxed names to be reused, thereby avoiding the cost of
computing all of the relaxed variants of the target name a second time.

Closes gh-4775