Make FileUtils available to plugin developers

If would be useful to plugin developers if

org.gradle.internal.FileUtils

from

./subprojects//base-services/src/main/java/org/gradle/internal/FileUtils.java

is made available for their use. (The toSafeFilenamemethod is of particular interest).

Although we can use it now, the fact that it has internal in the namespace means that we are curretnly relying on somehting that could theoretically cahnge between versions without warning.