This PR represents the initial phase of Modularizing Elasticsearch (with
Java Modules).

This initial phase modularizes the core of the Elasticsearch server
with Java Modules, which is then used to load and configure extension
components atop the server. Only a subset of extension components are
modularized at this stage (other components come in a later phase).
Components are loaded dynamically at runtime with custom class loaders
(same as is currently done). Components with a module-info.class are
defined to a module layer.

This architecture is somewhat akin to the Modular JDK, where
applications run on the classpath. In the analogy, the Elasticsearch
server modules are the platform (thus are always resolved and present),
while components without a module-info.class are non-modular code
running atop the Elasticsearch server modules. The extension components
cannot access types from non-exported packages of the server modules, in
the same way that classpath applications cannot access types from
non-exported packages of modules from the JDK. Broadly, the core
Elasticseach java modules simply "wrap" the existing packages and export
them. There are opportunites to export less, which is best done in more
narrowly focused follow-up PRs.

The Elasticsearch distribution startup scripts are updated to put jars
on the module path (the class path is empty), so the distribution will
run the core of the server as java modules. A number of key components
have been retrofitted with module-info.java's too, and the remaining
components can follow later. Unit and functional tests run as
non-modular (since they commonly require package-private access), while
higher-level integration tests, that run the distribution, run as
modular.

relates #78744