Cleanup: Move blend file path utilities out of blenloader module

These are not really about reading or writing .blend files, they are
general utilities for file-names/paths. Having to link to the
blendloader library just for these utilities is annoying.
Moved them to `BKE_blendfile.h` now, in agreement with Bastien.

Pull Request: #105825

source/blender/blenkernel/BKE_blendfile.h

@@ -19,6 +19,27 @@ struct ReportList;
 struct UserDef;
 struct bContext;
 
+/**
+ * Check whether given path ends with a blend file compatible extension
+ * (`.blend`, `.ble` or `.blend.gz`).
+ *
+ * \param str: The path to check.
+ * \return true is this path ends with a blender file extension.
+ */
+bool BKE_has_bfile_extension(const char *str);
+/**
+ * Try to explode given path into its 'library components'
+ * (i.e. a .blend file, id type/group, and data-block itself).
+ *
+ * \param path: the full path to explode.
+ * \param r_dir: the string that'll contain path up to blend file itself ('library' path).
+ * WARNING! Must be #FILE_MAX_LIBEXTRA long (it also stores group and name strings)!
+ * \param r_group: the string that'll contain 'group' part of the path, if any. May be NULL.
+ * \param r_name: the string that'll contain data's name part of the path, if any. May be NULL.
+ * \return true if path contains a blend file.
+ */
+bool BKE_library_path_explode(const char *path, char *r_dir, char **r_group, char **r_name);
+
 /**
  * Shared setup function that makes the data from `bfd` into the current blend file,
  * replacing the contents of #G.main.

source/blender/blenkernel/intern/blendfile.cc

@@ -62,6 +62,78 @@
 #  include "BPY_extern.h"
 #endif
 
+/* -------------------------------------------------------------------- */
+/** \name Blend/Library Paths
+ * \{ */
+
+bool BKE_has_bfile_extension(const char *str)
+{
+  const char *ext_test[4] = {".blend", ".ble", ".blend.gz", nullptr};
+  return BLI_path_extension_check_array(str, ext_test);
+}
+
+bool BKE_library_path_explode(const char *path, char *r_dir, char **r_group, char **r_name)
+{
+  /* We might get some data names with slashes,
+   * so we have to go up in path until we find blend file itself,
+   * then we know next path item is group, and everything else is data name. */
+  char *slash = nullptr, *prev_slash = nullptr, c = '\0';
+
+  r_dir[0] = '\0';
+  if (r_group) {
+    *r_group = nullptr;
+  }
+  if (r_name) {
+    *r_name = nullptr;
+  }
+
+  /* if path leads to an existing directory, we can be sure we're not (in) a library */
+  if (BLI_is_dir(path)) {
+    return false;
+  }
+
+  strcpy(r_dir, path);
+
+  while ((slash = (char *)BLI_path_slash_rfind(r_dir))) {
+    char tc = *slash;
+    *slash = '\0';
+    if (BKE_has_bfile_extension(r_dir) && BLI_is_file(r_dir)) {
+      break;
+    }
+    if (STREQ(r_dir, BLO_EMBEDDED_STARTUP_BLEND)) {
+      break;
+    }
+
+    if (prev_slash) {
+      *prev_slash = c;
+    }
+    prev_slash = slash;
+    c = tc;
+  }
+
+  if (!slash) {
+    return false;
+  }
+
+  if (slash[1] != '\0') {
+    BLI_assert(strlen(slash + 1) < BLO_GROUP_MAX);
+    if (r_group) {
+      *r_group = slash + 1;
+    }
+  }
+
+  if (prev_slash && (prev_slash[1] != '\0')) {
+    BLI_assert(strlen(prev_slash + 1) < MAX_ID_NAME - 2);
+    if (r_name) {
+      *r_name = prev_slash + 1;
+    }
+  }
+
+  return true;
+}
+
+/** \} */
+
 /* -------------------------------------------------------------------- */
 /** \name Blend File IO (High Level)
  * \{ */