Add plugin for Swift Package Manager

[mattt]

This PR adds SBOM generation support for Swift Package Manager, a package manager for the Swift programming language.

To try this out, install Swift and run the following commands:

$ git clone https://github.com/apple/example-package-dealer.git path/to/example-package-dealer
$ swift build --package-path path/to/example-package-dealer
$ ARGS="--path path/to/example-package-dealer" make generate

bom-swift.spdx:

SPDXVersion: SPDX-2.2
DataLicense: CC0-1.0
SPDXID: SPDXRef-DOCUMENT
DocumentName: dealer-2.0.0
DocumentNamespace: http://spdx.org/spdxpackages/dealer-2.0.0-71cfdcae-458d-4428-bc56-4a866f34f79e
Creator: Tool: spdx-sbom-generator-source-code
Created: 2021-07-26T17:27:02Z

##### Package representing the dealer

PackageName: dealer
SPDXID: SPDXRef-Package-dealer
PackageVersion: 2.0.0
PackageSupplier: NOASSERTION
PackageDownloadLocation: NOASSERTION
FilesAnalyzed: false
PackageChecksum: SHA1: 22e26b7ba613a2de93b0d0e906e31aed5b41e6a2

PackageHomePage: NOASSERTION
PackageLicenseConcluded: NOASSERTION
PackageLicenseDeclared: NOASSERTION
PackageCopyrightText: NOASSERTION
PackageLicenseComments: NOASSERTION
PackageComment: NOASSERTION

Relationship: SPDXRef-DOCUMENT DESCRIBES SPDXRef-Package-dealer 

##### Package representing the DeckOfPlayingCards

PackageName: DeckOfPlayingCards
SPDXID: SPDXRef-Package-DeckOfPlayingCards-3.0.4
PackageVersion: 3.0.4
PackageSupplier: NOASSERTION
PackageDownloadLocation: git+https://github.com/apple/example-package-deckofplayingcards.git
FilesAnalyzed: false
PackageChecksum: SHA1: 2c0e5ac3e10216151fc78ac1ec6bd9c2c0111a3a

PackageHomePage: https://github.com/apple/example-package-deckofplayingcards
PackageLicenseConcluded: NOASSERTION
PackageLicenseDeclared: NOASSERTION
PackageCopyrightText: NOASSERTION
PackageLicenseComments: NOASSERTION
PackageComment: NOASSERTION