This adds the following new fields:

 - `.compiler-id`
 - `.os`
 - `.arch`
 - `.install-plan[].pkg-name`
 - `.install-plan[].pkg-version`
 - `.install-plan[].pkg-src-sha256`

Having separate "pkg-{name,version}" fields is desirable because the "id"-field
is actually a UnitId which is supposed to be an opaque id which may not
always be easily convertible into a PackageId via string operations.

The "pkg-src-sha256" is generally useful to uniquely identify a
source-tarball (when the package-id is not exact enough).

Finally, the compiler-id/os/arch information is needed to determine the
values of the respective impl()/os()/arch() predicates used in
conditionals in .cabal files.

Here's an example demonstrating the new fields:

    {
      "cabal-version": "1.25.0.0",
      "cabal-lib-version": "1.25.0.0",
      "compiler-id": "ghc-8.0.1",
      "os": "linux",
      "arch": "x86_64",
      "install-plan": [
	{
	  "type": "configured",
	  "id": "StateVar-1.1.0.4-048d5d25d5813f17f152f9e766fe3ea0ef6f5317439aa115f3fc24e53a0e3c17",
	  "pkg-name": "StateVar",
	  "pkg-version": "1.1.0.4",
	  "flags": {},
	  "style": "global",
	  "pkg-src-sha256": "7ad68decb5c9a76f83c95ece5fa13d1b053e4fb1079bd2d3538f6b05014dffb7",
	  "depends": [
	    "base-4.9.0.0",
	    "stm-2.4.4.1-2c7789312d2396a91db173e1ae8cf3a9af3dd18de42bb1021c4978fbab49c191",
	    "transformers-0.5.2.0"
	  ],
	  "exe-depends": [],
	  "component-name": "lib"
	},
        {
          "type": "pre-existing",
          "id": "rts",
          "pkg-name": "rts",
          "pkg-version": "1.0",
          "depends": []
        },
	{
	  "type": "configured",
	  "id": "hackage-matrix-builder3-0.3-inplace-matrix-lib",
	  "pkg-name": "hackage-matrix-builder3",
	  "pkg-version": "0.3",
	  "component-name": "lib:matrix-lib"
	  ...
	}
	...
    }