build: switch to the Zig package manager

No more git submodules!

PACKAGING.md

@@ -26,6 +26,45 @@ and is only compatible with that release and any patch releases. At the time
 of writing for example river is compatible with Zig 0.9.0 and 0.9.1 but
 not Zig 0.8.0 or 0.10.0.
 
+## Zig Package Manager
+
+River uses the built-in Zig package manager for its (few) Zig dependencies.
+By default, running `zig build` will fetch river's Zig dependencies from the
+internet and store them in the global zig cache before building river. Since
+accessing the internet is forbidden or at least frowned upon by most distro
+packaging infrastructure, there are ways to fetch the Zig dependencies in a
+separate step before building river:
+
+1. Fetch step with internet access:
+
+   For each package in the `build.zig.zon` manifest file run the following command
+   with the tarball URL in the `build.zig.zon`:
+
+   ```
+   zig fetch --global-cache-dir /tmp/foobar $URL
+   ```
+
+   This command will download and unpack the tarball, hash the contents of the
+   tarball, and store the contents in the `/tmp/foobar/p/$HASH` directory. This
+   hash should match the corresponding hash field in the `build.zig.zon`.
+
+2. Build step with no internet access:
+
+   The `--system` flag for `zig build` takes a path to an arbitrary directory in
+   which zig packages stored in subdirectories matching their hash can be found.
+
+   ```
+   zig build --system /tmp/foobar/p/ ...
+   ```
+
+   This flag will disable all internet access and error if a package is not found
+   in the provided directory.
+
+It is also possible for distros to distribute Zig package manager packages as
+distro packages, although there are still some rough edges as the support for
+this is not yet mature. See this patchset for Chimera Linux for an example of
+how this can work: https://github.com/chimera-linux/cports/pull/1395
+
 ## Build options
 
 River is built using the Zig build system. To see all available build