Resolve symlinks when fetching file contents

[wagoodman]

It was noticed that file-classifications was empty for /bin/busybox when cataloging the busybox image with the classifications cataloger enabled. Since we have a busybox-binary classifier, this was unexpected. Primarily this is due to the fact that /bin/busybox is a symlink to /bin/[ and the classifier filters files considered by filename (in this case **/busybox, which /bin/[ would never match on). The existing behavior of source.FileResolver when using FetchContents() is to only return contents for unresolved links.

This PR changes this behavior to resolve symlinks first (for all resolver implementations) before attempting to fetch contents. As a result the file-digests cataloger has been adjusted to only report digests for regular files (don't follow links), which is closest to today's behavior.

Open question: should we have the digests cataloger report digests for resolved paths? or only regular files?
Answer: No --it would be confusing to see a symlink have content, and this would be redundant (the real file will be additionally be in the list and the link destination is already reported)

Additional changes:

• bumped stereoscope to pull in mimetype fix for empty io.Readers
• the cwd and root directories used when creating the directory resolver are evaluated for possible access via links, such that normalizing response paths is consistent with the real path and not the link path

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

name                                                       old time/op    new time/op    delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2              1.30ms ± 1%    1.82ms ± 1%  +40.31%  (p=0.016 n=4+5)
ImagePackageCatalogers/python-package-cataloger-2            2.97ms ± 0%    4.20ms ±17%  +41.34%  (p=0.008 n=5+5)
ImagePackageCatalogers/php-composer-installed-cataloger-2    1.03ms ± 1%    1.40ms ± 2%  +36.19%  (p=0.008 n=5+5)
ImagePackageCatalogers/javascript-package-cataloger-2         802µs ± 2%    1091µs ± 1%  +36.04%  (p=0.008 n=5+5)
ImagePackageCatalogers/dpkgdb-cataloger-2                     933µs ± 1%    1275µs ± 2%  +36.71%  (p=0.008 n=5+5)
ImagePackageCatalogers/rpmdb-cataloger-2                      846µs ± 2%    1141µs ± 5%  +34.93%  (p=0.008 n=5+5)
ImagePackageCatalogers/java-cataloger-2                      23.4ms ± 4%    29.8ms ± 6%  +27.45%  (p=0.008 n=5+5)
ImagePackageCatalogers/apkdb-cataloger-2                     1.28ms ± 2%    1.74ms ± 2%  +35.86%  (p=0.008 n=5+5)
ImagePackageCatalogers/go-module-binary-cataloger-2          1.98µs ± 1%    2.57µs ±10%  +29.94%  (p=0.008 n=5+5)

name                                                       old alloc/op   new alloc/op   delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2               252kB ± 0%     253kB ± 0%   +0.32%  (p=0.008 n=5+5)
ImagePackageCatalogers/python-package-cataloger-2            1.06MB ± 0%    1.07MB ± 0%   +0.30%  (p=0.008 n=5+5)
ImagePackageCatalogers/php-composer-installed-cataloger-2     253kB ± 0%     253kB ± 0%     ~     (p=0.151 n=5+5)
ImagePackageCatalogers/javascript-package-cataloger-2         208kB ± 0%     208kB ± 0%   +0.21%  (p=0.008 n=5+5)
ImagePackageCatalogers/dpkgdb-cataloger-2                     254kB ± 0%     255kB ± 0%   +0.34%  (p=0.008 n=5+5)
ImagePackageCatalogers/rpmdb-cataloger-2                      236kB ± 0%     236kB ± 0%   +0.24%  (p=0.008 n=5+5)
ImagePackageCatalogers/java-cataloger-2                      4.19MB ± 0%    4.20MB ± 0%     ~     (p=0.222 n=5+5)
ImagePackageCatalogers/apkdb-cataloger-2                     1.30MB ± 0%    1.30MB ± 0%   +0.23%  (p=0.008 n=5+5)
ImagePackageCatalogers/go-module-binary-cataloger-2            608B ± 0%      608B ± 0%     ~     (all equal)

name                                                       old allocs/op  new allocs/op  delta
ImagePackageCatalogers/ruby-gemspec-cataloger-2               6.33k ± 0%     6.33k ± 0%     ~     (p=0.444 n=5+5)
ImagePackageCatalogers/python-package-cataloger-2             21.4k ± 0%     21.4k ± 0%     ~     (p=0.603 n=5+5)
ImagePackageCatalogers/php-composer-installed-cataloger-2     7.25k ± 0%     7.25k ± 0%     ~     (p=0.095 n=5+4)
ImagePackageCatalogers/javascript-package-cataloger-2         5.36k ± 0%     5.36k ± 0%     ~     (all equal)
ImagePackageCatalogers/dpkgdb-cataloger-2                     7.10k ± 0%     7.10k ± 0%     ~     (all equal)
ImagePackageCatalogers/rpmdb-cataloger-2                      6.82k ± 0%     6.82k ± 0%     ~     (all equal)
ImagePackageCatalogers/java-cataloger-2                       86.8k ± 0%     86.8k ± 0%     ~     (p=0.587 n=5+5)
ImagePackageCatalogers/apkdb-cataloger-2                      7.37k ± 0%     7.37k ± 0%     ~     (p=1.000 n=5+5)
ImagePackageCatalogers/go-module-binary-cataloger-2            14.0 ± 0%      14.0 ± 0%     ~     (all equal)