Let ingest_tarball.sh support custom installation prefixes

[Neves-P]

With this PR we are in the last (?) steps to generalize support for custom installation prefixes, following from EESSI/software-layer-scripts#239 and EESSI/software-layer-scripts#295

This is still a draft, as the verions subpath argument must still be obtained from the tarball metadata. This is currently not encoded anywhere we can easily gather at the ingestion stage.

• Added optional third argument for the repository-relative versions subpath:

  ingest-tarball.sh <repository> <tarball> [versions subpath]

• Defaults to versions, preserving standard repository behavior.
• Removed the dev.eessi.io-specific filename inference.
• Added validation preventing absolute paths and ./.. path components.
• Ensured Lmod searches the custom subpath in the target repository while still using software.eessi.io/versions when falling back.

[boegel]

@Neves-P Can you clarify why you're closing this?

[Neves-P]

@boegel sorry about that! That was unintentional, I accidentally deleted the wrong branch on my fork!

[Neves-P]

I don't get why scripts/automatic-cvmfs-named-snapshot/automatic-cvmfs-named-snapshot.sh is in the list of changed files because it isn't in any of the commits... 🤔