Internalize majority of cmd package (#2533)

* internalize majority of cmd package and migrate integration tests

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* add internal api encoder

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* create internal representation of all formats

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* export capability to get default encoders

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* restore test fixtures

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

cmd/syft/internal/test/integration/test-fixtures/image-sbom-cataloger/Dockerfile

@@ -0,0 +1,3 @@
+FROM scratch
+COPY go.mod /
+COPY test.spdx.json /

cmd/syft/internal/test/integration/test-fixtures/image-sbom-cataloger/go.mod

@@ -0,0 +1,8 @@
+module github.com/anchore/syft
+
+go 1.14
+
+require (
+	github.com/anchore/packageurl-go v0.1.1-0.20220428202044-a072fa3cb6d7
+	github.com/bmatcuk/doublestar v1.3.1
+)

cmd/syft/internal/test/integration/test-fixtures/image-sbom-cataloger/test.spdx.json

@@ -0,0 +1,39 @@
+{
+ "SPDXID": "SPDXRef-DOCUMENT",
+ "name": "test/integration/test-fixtures/image-sbom-cataloger",
+ "spdxVersion": "SPDX-2.2",
+ "creationInfo": {
+  "created": "2022-08-18T05:23:38.066146511Z",
+  "creators": [
+   "Organization: Anchore, Inc",
+   "Tool: syft-0.53.0"
+  ],
+  "licenseListVersion": "3.17"
+ },
+ "dataLicense": "CC0-1.0",
+ "documentNamespace": "https://anchore.com/syft/dir/test/integration/test-fixtures/image-sbom-cataloger-057e7aa2-332c-4b50-bbd1-a17dd62ddce4",
+ "packages": [
+  {
+   "SPDXID": "SPDXRef-45209ca0cdcbafa",
+   "name": "github.com/bmatcuk/doublestar",
+   "licenseConcluded": "NONE",
+   "downloadLocation": "NOASSERTION",
+   "externalRefs": [
+    {
+     "referenceCategory": "SECURITY",
+     "referenceLocator": "cpe:2.3:a:bmatcuk:doublestar:v1.3.1:*:*:*:*:*:*:*",
+     "referenceType": "cpe23Type"
+    },
+    {
+     "referenceCategory": "PACKAGE_MANAGER",
+     "referenceLocator": "pkg:golang/github.com/bmatcuk/doublestar@v1.3.1",
+     "referenceType": "purl"
+    }
+   ],
+   "filesAnalyzed": false,
+   "licenseDeclared": "NONE",
+   "sourceInfo": "acquired package info from go module information: go.mod",
+   "versionInfo": "v1.3.1"
+  }
+ ]
+}