Improve cache system

Fix #3464

- Invalidate the cache if the target language changed
- Always delete the fable_modules folder if the cache is invalide

src/Fable.Cli/ProjectCracker.fs

@@ -722,6 +722,7 @@ let getFullProjectOpts (opts: CrackerOptions) =
 
             cacheInfo.Version = Literals.VERSION
             && cacheInfo.Exclude = opts.Exclude
+            && cacheInfo.FableOptions.Language = opts.FableOptions.Language
             && (
                 [
                     cacheInfo.ProjectPath
@@ -789,6 +790,11 @@ let getFullProjectOpts (opts: CrackerOptions) =
     | None ->
         let projRefs, mainProj = retryGetCrackedProjects opts
 
+        // The cache was considered outdated / invalid so it is better to make
+        // make sure we have are in a clean state
+        if IO.Directory.Exists(opts.FableModulesDir) then
+            IO.Directory.Delete(opts.FableModulesDir, true)
+
         let fableLibDir, pkgRefs =
             match opts.FableOptions.Language with
             | Python -> copyFableLibraryAndPackageSourcesPy opts mainProj.PackageReferences

src/Fable.Cli/RELEASE_NOTES.md

@@ -5,6 +5,8 @@
 * Remove fable-py support
 * Fix #3461: Don't default to javascript if the language requested by the user is unknown (help detect typo)
 * Improve --help message for the --lang section
+* Fix #3464: Invalidate cache when the target language changes
+* Always delete the `fable_modules` folder when the cache is invalidated
 
 ### 4.1.4