New API introduced: std.Build.addModule

This function exposes a zig module with the given name, which can be
used by packages that depend on this one via std.Build.Dependency.module.

std.Build.Pkg and related functionality is deleted. Every use case has a
straightforward upgrade path using the new Module struct.

std.Build.OptionsStep.getPackage is replaced by
std.Build.OptionsStep.createModule.

std.Build.CompileStep.addPackagePath is replaced by
std.Build.CompileStep.addAnonymousModule.

This partially addresses #14307 by renaming some of the instances of
"package" to "module".

Closes #14278