aconfig: restrict valid namespace and flag names

The namespace and flag names will be used as identifiers in the
auto-generated code. Place restrictions on what constitutes a valid
name.

Valid identifiers are those that match /[a-z][a-z0-9_]/. aconfig
explicitly does not implement any automatic translation to make names
valid identifiers: this sidesteps potential conflicts such as "foo.bar"
and "foo_bar" mapping to the same name if dots were translated to
underscores.

Bug: b/284252015
Test: atest aconfig.test
Change-Id: I38d005a74311e5829e540063404d1565071e6e96
diff --git a/tools/aconfig/src/main.rs b/tools/aconfig/src/main.rs
index b60909b..1d2ec95 100644
--- a/tools/aconfig/src/main.rs
+++ b/tools/aconfig/src/main.rs
@@ -26,6 +26,7 @@
 
 mod aconfig;
 mod cache;
+mod codegen;
 mod codegen_cpp;
 mod codegen_java;
 mod codegen_rust;