[B] Effect, xp and clear commands can target incorrect players - Fixes BUKKIT-5356

**The Issue:**
For 3 commands (effect, xp and clear) the player entered may not always be the player that receives the effect of the command. The code is told to find a player with the name, where it should find the exact player. One example is the clear command, an OP may accidentally remove a players inventory causing havoc.

**Justification for this PR:**
This PR prevents possible accidental mishaps, at the loss of being able to shortcut names (eg having to put stuntguy instead of stuntguy3000). It is crucial that the user entered is the user that receives the effect of the command, because the three commands which this effects can cause damage (mainly the clear inventory command).

This also helps to mimic vanilla behavior.

**JIRA Ticket:** https://bukkit.atlassian.net/browse/BUKKIT-5356

**Testing Material**
Compiled changes from local fork and tested.

**Associated Bukkit Ticket:** https://github.com/Bukkit/Bukkit/pull/1019