Let symlinks be backed by path.Parser

[EdSchouten]

This change is necessary to catch up with the API changes introduced in buildbarn/bb-remote-execution#216. It removes any implementations of, and calls to VirtualReadLink(), and uses VirtualGetAttributes(AttributesMaskSymlinkTarget) instead.

With this change applied, symlinks created in the virtual file system are always normalized.

$ ln -s //////../foo//bar/./baz
$ ls -l baz
lrwxrwxrwx  9999 nobody  nobody  12 Jan  1  2000 baz -> /foo/bar/baz

[aspect-workflows]

Test

4 test targets passed

Targets

//pkg/blobstore:blobstore_test [k8-fastbuild]                         71ms
//pkg/cas:cas_test [k8-fastbuild]                                     167ms
//pkg/filesystem/virtual:virtual_test [k8-fastbuild]                  205ms
//pkg/outputpathpersistency:outputpathpersistency_test [k8-fastbuild] 189ms