Don't replace : with - in docker SPDX namespaces

[aethanol]

What would you like to be added:
In #649, https://github.com/anchore/syft/blob/main/internal/formats/common/spdxhelpers/document_name.go#L26 was introduced which replaces : with - in namespaces. This impacts the output name of docker SPDX.
Ideally these docker names like IMAGE[:TAG|@DIGEST] would be the same as outputted by syft.

Why is this needed:
We are having to manually parse this on ingestion time, back to : so we can correlate docker images with their syft scans.

[kzantow]

For document name, we should not be "cleaning" the name -- it is a raw string. Check the following functions, it looks like we should NOT be calling cleanName:

• https://github.com/anchore/syft/blob/main/syft/formats/common/spdxhelpers/document_namespace.go#L19
• https://github.com/anchore/syft/blob/main/syft/formats/common/spdxhelpers/document_name.go#L10