Return declared licence from ParseResult

[andrew]

ParseResult already carries the package's own Name and Version from the manifest. Most of the same formats also declare a licence in the same document node, and both scancode --package and licensee read it as the primary declared-licence signal.

Proposed fields:

type ParseResult struct {
    // ...
    // Licenses holds the package's declared licence values, verbatim as
    // written in the manifest. Empty when the format has no licence field
    // or none is set. Scalar formats produce a single-element slice.
    Licenses []string
    // LicenseFile is a manifest-relative path to a licence file when the
    // format declares one instead of, or as well as, an expression.
    LicenseFile string
}

Raw values only; callers normalise via github.com/git-pkgs/spdx if they want an SPDX expression. That keeps this package free of the spdx dependency and its regex normaliser.

Formats and their fields:

format	licence expression	array form	licence file
package.json	license (string or {type,url})	deprecated licenses: [{type,url}]
Cargo.toml	[package].license		[package].license-file
pyproject.toml	[project].license (PEP 639 string) or {text}	[project].classifiers License :: entries	[project].license-files, or {file}
setup.cfg / setup.py	license	classifiers	license_files
*.gemspec	s.license	s.licenses
*.podspec	s.license (string or {:type, :file})		:file key
composer.json	license (string)	license (array)
pom.xml	<licenses><license><name>	multiple <license> elements	<url> when it points at a file
*.nuspec	<license type="expression">		<license type="file">
*.cabal	license		license-file / license-files
DESCRIPTION (R)	License:	`	`-separated
pubspec.yaml	none (Dart uses LICENSE file)
go.mod	none
mix.exs	licenses: in package/0	list
*.opam	license:	list
elm.json	license
bower.json	license	array

Cost per manifest is one more field read from a node the parser already decodes; no extra I/O or parse pass. The DSL formats (gemspec, podspec, mix.exs) need one more scrape pattern alongside the existing name/version ones.

Consumers: git pkgs licenses for the declared/detected reconciliation, and git-pkgs/licenses (the scanner) which can read LicenseFile and match its text.