build: refactor and decouple pkg builds from main build (#742)

This refactor decouples the build phases of the Vere submodules (c3,
ent, ur, noun, and vere) from the main Urbit binary Zig build by moving
them into separate Zig builds within each respective pkg directory.

This change is intended to resolve certain Zig cache issues where the
linker fails to locate newly added symbols in submodules when these
symbols are used in dependent modules.

INSTALL.md

@@ -69,9 +69,10 @@ Supported values:
 - soon
 - edge
 
-#### `-Dflags=[string]`
-Provide additional compiler flags. These propagate to all build artifacts and
-dependencies.
+#### `-Dcopt=[list]`
+Provide additional compiler flags. These propagate to all build artifacts.
+
+Example: `zig build -Dcopt="-g" -Dcopt="-fno-sanitize=all"`
 
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