/usr/lib/R/site-library/fastcluster/INDEX is in r-cran-fastcluster 1.1.13-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 | fastcluster             Fast hierarchical, agglomerative clustering
                        routines for R and Python
hclust                  Fast hierarchical, agglomerative clustering of
                        dissimilarity data
hclust.vector           Fast hierarchical, agglomerative clustering of
                        vector data
 |