Add ability to generate a c header file from scratch, and then both
compile with it and install it if needed.
Example:
```zig
const avconfig_h = b.addConfigHeader(.{ .path = "libavutil/avconfig.h" }, .generated, .{
.AV_HAVE_BIGENDIAN = 0, // TODO: detect based on target
.AV_HAVE_FAST_UNALIGNED = 1, // TODO: detect based on target
});
lib.addConfigHeader(avconfig_h);
lib.installConfigHeader(avconfig_h);
```
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
Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.
