Use single character variant of String.replace when applicable

String.replace('a', 'b') is conceivably slightly more efficient than String.replace("a", "b").

There is a couple of instances of the later variant found in
src/java.base/share/classes/jdk/internal/module/Resources.java
and
src/java.base/share/classes/jdk/internal/module/ModulePath.java

Reported via core-libs-dev mailing alias.