internal/pkg: enable cache access during store

This is still not ideal as it makes entry into Store sequential. This will be improved after more usage code is written.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -144,7 +144,10 @@ type Cache struct {
 	// Directory where all [Cache] related files are placed.
 	base *check.Absolute
 
-	// Synchronises access to public methods.
+	// Protects the Store critical section.
+	storeMu sync.Mutex
+
+	// Synchronises access to most public methods.
 	mu sync.RWMutex
 }
 
@@ -208,8 +211,8 @@ func (c *Cache) Store(
 	err error,
 ) {
 	pathname = c.pathnameIdent(&id)
-	c.mu.Lock()
+	c.storeMu.Lock()
-	defer c.mu.Unlock()
+	defer c.storeMu.Unlock()
 
 	_, err = os.Lstat(pathname.String())
 	if err == nil || !errors.Is(err, os.ErrNotExist) {