refactor(tvix/derivation): remove DOT_FILE_EXT const

This is used in few enough places to just inline it. It felt a bit alien
in the ATerm construction aswell.

write.rs now pleasantly only includes tokens that occur in the ATerm
representation.

Change-Id: I524f8d6c1ce9057ff7fd16c6c3efd98467040a44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7911
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/derivation/src/derivation.rs

@@ -64,15 +64,7 @@ fn build_store_path(
     };
     let compressed = compress_hash(&digest, 20);
     if is_derivation {
-        StorePath::from_string(
-            format!(
-                "{}-{}{}",
-                NIXBASE32.encode(&compressed),
-                name,
-                write::DOT_FILE_EXT,
-            )
-            .as_str(),
-        )
+        StorePath::from_string(format!("{}-{}.drv", NIXBASE32.encode(&compressed), name).as_str())
     } else {
         StorePath::from_string(format!("{}-{}", NIXBASE32.encode(&compressed), name,).as_str())
     }

tvix/derivation/src/validate.rs

@@ -1,4 +1,4 @@
-use crate::{derivation::Derivation, write::DOT_FILE_EXT, DerivationError};
+use crate::{derivation::Derivation, DerivationError};
 use tvix_store::store_path::StorePath;
 
 impl Derivation {
@@ -60,7 +60,7 @@ impl Derivation {
                 ));
             }
 
-            if !input_derivation_path.ends_with(DOT_FILE_EXT) {
+            if !input_derivation_path.ends_with(".drv") {
                 return Err(DerivationError::InvalidInputDerivationPrefix(
                     input_derivation_path.to_string(),
                 ));

tvix/derivation/src/write.rs

@@ -16,8 +16,6 @@ pub const BRACKET_CLOSE: char = ']';
 pub const COMMA: char = ',';
 pub const QUOTE: char = '"';
 
-pub const DOT_FILE_EXT: &str = ".drv";
-
 fn write_array_elements(
     writer: &mut impl Write,
     quote: bool,