[API] promote org.eclipse.jdt.internal.debug.core.breakpoints.ValidBreakpointLocationLocator into an API

This classes has usages in vscode java debug plugin to locate elements using the AST tree with given line numbers. providing a better API for this from jdt debug will enable this to be used in such plugins without depending on internals. Also this might be use full for other types of plugins. May be this could be moved to jdt.core since this doesn't have any debug related business logic.

@SarikaSinha WDYT ?