utils/spdx: fix invalid SPDX syntax for symbols

[cho-m]

Also use more common uppercase operators for backwards compatibility.

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https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/#d1-overview - Spaces are invalid (e.g. Cannot Represent) and non-official SPDX entries must at least have LicenseRef- (could also have DocumentRef-<name>:LicenseRef- but I doubt we would ever need this in Homebrew).

SPDX expressions operators should be case-sensitive uppercase - https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/#d2-case-sensitivity (In PR, for backwards compatibility with existing JSON API, need to match both for now). EDIT: Lowercase is allowed in newer spec, but uppercase is more common so okay either way but switching seems better for compatibility.

WITH allows for a compound expression

[MikeMcQuaid]

Makes sense, thanks @cho-m!

[cho-m]

Updated to use LicenseRef-Homebrew-public-domain and LicenseRef-Homebrew-cannot-represent. Main reason is for simplicity of translation (since we could add other "allowed" symbols) and there isn't really a standard for these.

For public domain, if any identifier becomes well adopted, then we can consider switching. Currently, it is all over the place:

• LicenseRef-Public-Domain - Python? https://peps.python.org/pep-0639/appendix-mapping-classifiers/
• LicenseRef-PublicDomain - Haskell?
• LicenseRef-PD-2 (-d, -hd, -hp) - https://cr.yp.to/spdx.html
• LicenseRef-Fedora-Public-Domain - Fedora
• LicenseRef-scancode-public-domain - ScanCode LicenseDB

[MikeMcQuaid]

Thanks again @cho-m!