Denote container package type in SPDX docs

[rnjudge]

Describe the Feature
SPDX 2.3 is proposing adding a package purpose field which could be used to denote when the package is a container (among other package types). Once the proposal is agreed upon and merged, add the package purpose type to Tern SPDX reports.

Depends on: spdx/spdx-spec#622

[rjb4standards]

Update proposed Package description materials as needed to support this item ref: spdx/spdx-spec#628

[rnjudge]

As per spdx/spdx-spec#721, the packagePurpose is now primaryPackagePurpose. This means that in SPDX docs generated by Tern this field should be added as follows:

Tag Value:

PrimaryPackagePurpose: CONTAINER

JSON, under the top-level container "package" entry:

"primaryPackagePurpose": "CONTAINER"