Support deleting the manifest for a referrer

[sudo-bmitch]

Currently, when the manifest for a referrer is deleted, future queries to _oci/artifacts/referrers return a 500:

INFO[0004] response completed                            go.version=go1.17.11 http.request.host="localhost:5050" http.request.id=071c0a6d-eec9-45a1-b0ae-3a98dca2ba49 http.request.method=DELETE http.request.remoteaddr="[::1]:48114" http.request.uri="/v2/test-cosign/regctl/manifests/sha256:243eadc1580231b803f5367bf8694af9a6ad747aed72afb3e8e86faca6329569" http.request.useragent="regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)" http.response.duration=2.894168ms http.response.status=202 http.response.written=0 instance.id=04d1639d-4914-4e4a-ab4e-4a600ea85e7a service=registry version=v2.7.0-1992-gd978c07e.m
::1 - - [12/Jun/2022:11:51:22 -0400] "DELETE /v2/test-cosign/regctl/manifests/sha256:243eadc1580231b803f5367bf8694af9a6ad747aed72afb3e8e86faca6329569 HTTP/1.1" 202 0 "" "regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)"                                                                                                                                                                                                
INFO[0010] response completed                            go.version=go1.17.11 http.request.host="localhost:5050" http.request.id=3089f7c6-3ad5-4035-9937-7475a080070b http.request.method=HEAD http.request.remoteaddr="[::1]:48116" http.request.uri=/v2/test-cosign/regctl/manifests/latest http.request.useragent="regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)" http.response.contenttype=application/vnd.docker.distribution.manifest.list.v2+json http.response.duration=2.163606ms http.response.status=200 http.response.written=2342 instance.id=04d1639d-4914-4e4a-ab4e-4a600ea85e7a service=registry version=v2.7.0-1992-gd978c07e.m
::1 - - [12/Jun/2022:11:51:28 -0400] "HEAD /v2/test-cosign/regctl/manifests/latest HTTP/1.1" 200 2342 "" "regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)"
ERRO[0010] response completed with error                 err.code=unknown err.detail="filesystem: unknown manifest name=test-cosign/regctl revision=sha256:243eadc1580231b803f5367bf8694af9a6ad747aed72afb3e8e86faca6329569" err.message="unknown error" go.version=go1.17.11 http.request.host="localhost:5050" http.request.id=a1c5c934-074a-48e9-8329-10da7b09513b http.request.method=GET http.request.remoteaddr="[::1]:48116" http.request.uri="/v2/test-cosign/regctl/_oci/artifacts/referrers?digest=sha256%3A169c32bea80d19ac2f1ed999fc4286d4dfcee816bdccc4353b4f4f5723528126" http.request.useragent="regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)" http.response.contenttype=application/json http.response.duration=2.471606ms http.response.status=500 http.response.written=221 instance.id=04d1639d-4914-4e4a-ab4e-4a600ea85e7a service=registry vars.name=test-cosign/regctl version=v2.7.0-1992-gd978c07e.m
::1 - - [12/Jun/2022:11:51:28 -0400] "GET /v2/test-cosign/regctl/_oci/artifacts/referrers?digest=sha256%3A169c32bea80d19ac2f1ed999fc4286d4dfcee816bdccc4353b4f4f5723528126 HTTP/1.1" 500 221 "" "regclient/regctl (f9688deb0f34143dd2049ce3f825dc975a617921)"