SPDX Tag-Value conversion not handling files directly set on packages

[kzantow]

What happened:
When parsing SPDX tag-value, Syft does not read files if they are directly set on packages.

What you expected to happen:
Syft reads files from all locations the SPDX tools-golang places them in the model.

Steps to reproduce the issue:
Convert this SPDX document to another format:

SPDXVersion: SPDX-2.2
DataLicense: CC0-1.0
SPDXID: SPDXRef-DOCUMENT
DocumentName: Some-SBOM
DocumentNamespace: https://example.org/some/namespace
Creator: Organization: Some-organization
Creator: Tool: Some-tool Version: 1.0
Created: 2021-12-29T17:02:21Z
PackageName: Some-package
PackageVersion: 5.1.2
SPDXID: SPDXRef-Package-43c51b08-cc7e-406d-8ad9-34aa292d1157
PackageSupplier: Organization: Some-organization
PackageDownloadLocation: https://example.org/download/location
FilesAnalyzed: true
PackageLicenseInfoFromFiles: NOASSERTION
PackageVerificationCode: 23460C5559C8D4DE3F6504E0E84E844CAC8B1D95
PackageLicenseConcluded: NOASSERTION
PackageLicenseDeclared: NOASSERTION
PackageCopyrightText: NOASSERTION
PackageChecksum: SHA1: 23460C5559C8D4DE3F6504E0E84E844CAC8B1D95
FileName: Some-file-name
SPDXID: SPDXRef-99545d55-933d-4e08-9eb5-9d826111cb79
FileContributor: Some-file-contributor
FileType: BINARY
FileChecksum: SHA1: 23460C5559C8D4DE3F6504E0E84E844CAC8B1D95
LicenseConcluded: NOASSERTION
LicenseInfoInFile: NOASSERTION
FileCopyrightText: NOASSERTION

Environment:

• Output of syft version:
  Application: syft
  Version: 0.86.1
  JsonSchemaVersion: 10.0.0
  BuildDate: 2023-07-31T17:29:18Z
  GitCommit: e2f7bef
  GitDescription: [not provided]
  Platform: darwin/amd64
  GoVersion: go1.20.6
  Compiler: gc