Project structure reorganisation

[oxisto]

This PR does a slight project reorgansation after some discussions:

• Rename csaf-import module to csaf-retrieval to be more in line with the wording of the standard. Also our package was already called retrieval
• Moving Requirement and Role to the csaf-retrieval package, since these are only valid in the context of distribution and retrieving CSAF documents (cf. Section 7).
• This makes the csaf-validation a pure CSAF document validation package, restricting to the contents of Section 6 (and possibly 4, once we introduce 'profiles').

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...b/csaf/sbom/retrieval/requirements/Requirements.kt	66.66%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ
...kotlin/io/github/csaf/sbom/retrieval/CsafLoader.kt	100.00% <100.00%> (ø)
...tlin/io/github/csaf/sbom/retrieval/ResultCompat.kt	100.00% <ø> (ø)
.../io/github/csaf/sbom/retrieval/RetrievalContext.kt	100.00% <100.00%> (ø)
.../github/csaf/sbom/retrieval/RetrievedAggregator.kt	100.00% <100.00%> (ø)
...io/github/csaf/sbom/retrieval/RetrievedDocument.kt	100.00% <100.00%> (ø)
...io/github/csaf/sbom/retrieval/RetrievedProvider.kt	97.77% <100.00%> (ø)
...main/kotlin/io/github/csaf/sbom/retrieval/Utils.kt	100.00% <ø> (ø)
...otlin/io/github/csaf/sbom/retrieval/Validatable.kt	100.00% <100.00%> (ø)
...ub/csaf/sbom/retrieval/requirements/Requirement.kt	100.00% <ø> (ø)
...kotlin/io/github/csaf/sbom/retrieval/roles/Role.kt	100.00% <ø> (ø)
... and 4 more