Overhaul build and expose API through platform modules

[AnttiMK]

Removes a lot of unnecessary configuration by consolidating common plugins and publishing config to the root build file. Notably, a) removes the plugins block from the root build file, as the root isn't supposed to be a build in itself, and b) exposes the API module via api scope through platform modules so consumers (aka. Plan itself) can consume the module directly (no need to separately declare API module as a dependency; see POM below).

Resulting POM for PAL-bukkit

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<!-- This module was also published with a richer model, Gradle metadata,  -->
<!-- which should be used instead. Do not delete the following line which  -->
<!-- is to indicate to Gradle or any Gradle module metadata file consumer  -->
<!-- that they should prefer consuming it instead. -->
<!-- do_not_remove: published-with-gradle-metadata -->
<modelVersion>4.0.0</modelVersion>
<groupId>net.playeranalytics</groupId>
<artifactId>platform-abstraction-layer-bukkit</artifactId>
<version>5.2.0</version>
<dependencies>
  <dependency>
    <groupId>net.playeranalytics</groupId>
    <artifactId>platform-abstraction-layer-api</artifactId>
    <version>5.2.0</version>
    <scope>compile</scope>
  </dependency>
</dependencies>
</project>

Required for plan-player-analytics/Plan#3953. Pls publish artifacts (and increment version) after merge <3

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