Make path mappings an object

[jablko]

Same rationale as #57:

• It's written as if it's unique but isn't, because getAllUniqueValues() only does shallow (===) equality:

  DefinitelyTyped-tools/packages/definitions-parser/src/lib/definition-parser.ts

  Line 223 in d885fe0

  pathMappings: getAllUniqueValues<"pathMappings", PathMapping>(allTypesVersions, "pathMappings"),

• I'm not aware of any advantages of the array format. The path mappings are used to refine dependencies and npm allows at most one semver per dependency, so you can't represent anything legal in the array that you can't with an object.
• We end up using the array for object lookup anyway:

  • DefinitelyTyped-tools/packages/definitions-parser/src/check-parse-results.ts

    Line 84 in d885fe0

    const pathMappings = new Map<string, TypingVersion>(pkg.pathMappings.map(p => [p.packageName, p.version]));

  • https://github.com/microsoft/DefinitelyTyped-tools/pull/87/files#diff-2943710c23a3f82fe40e2a34fa51c393R132