fix(npm): improve declaration resolution for filename with different extensions (#19966)

postcss was importing `./index.js` from `./index.d.mts` where there also
existed a `./index.d.ts`.

Closes #19575

2 files changed, 33 insertions, 0 deletions

diff --git a/cli/tests/testdata/npm/file_dts_dmts_dcts/main.out b/cli/tests/testdata/npm/file_dts_dmts_dcts/main.out
new file mode 100644
index 000000000..c92043f8b
--- /dev/null
+++ b/cli/tests/testdata/npm/file_dts_dmts_dcts/main.out
@@ -0,0 +1,24 @@
+Download http://localhost:4545/npm/registry/@denotest/file-dts-dmts-dcts
+Download http://localhost:4545/npm/registry/@denotest/file-dts-dmts-dcts/1.0.0.tgz
+Check file:///[WILDCARD]/main.ts
+error: TS2322 [ERROR]: Type '5' is not assignable to type '"dts"'.
+const value1: Dts1 = 5;
+      ~~~~~~
+    at file:///[WILDCARD]
+
+TS2322 [ERROR]: Type '5' is not assignable to type '"mts"'.
+const value2: Mts1 = 5;
+      ~~~~~~
+    at file:///[WILDCARD]
+
+TS2322 [ERROR]: Type '5' is not assignable to type '"mts"'.
+const value3: Mts2 = 5;
+      ~~~~~~
+    at file:///[WILDCARD]
+
+TS2322 [ERROR]: Type '5' is not assignable to type '"cts"'.
+const value4: Cts1 = 5;
+      ~~~~~~
+    at file:///[WILDCARD]
+
+Found 4 errors.
diff --git a/cli/tests/testdata/npm/file_dts_dmts_dcts/main.ts b/cli/tests/testdata/npm/file_dts_dmts_dcts/main.ts
new file mode 100644
index 000000000..63686e2d3
--- /dev/null
+++ b/cli/tests/testdata/npm/file_dts_dmts_dcts/main.ts
@@ -0,0 +1,9 @@
+import { Value as Dts1 } from "npm:@denotest/file-dts-dmts-dcts/js";
+import { Value as Mts1 } from "npm:@denotest/file-dts-dmts-dcts";
+import { Value as Mts2 } from "npm:@denotest/file-dts-dmts-dcts/mjs";
+import { Value as Cts1 } from "npm:@denotest/file-dts-dmts-dcts/cjs";
+
+const value1: Dts1 = 5;
+const value2: Mts1 = 5;
+const value3: Mts2 = 5;
+const value4: Cts1 = 5;