…yPath and removes functions that take literal paths instead of LazyPath.

…esolves ziglang#14971

* introduce LazyPath.cwd_relative variant and use it for --zig-lib-dir. closes ziglang#12685
* move overrideZigLibDir and setMainPkgPath to options fields set once
  and then never mutated.
* avoid introducing Build/util.zig
* use doc comments for deprecation notices so that they show up in
  generated documentation.
* introduce InstallArtifact.Options, accept it as a parameter to
  addInstallArtifact, and move override_dest_dir into it. Instead of
  configuring the installation via Compile step, configure the
  installation via the InstallArtifact step. In retrospect this is
  obvious.
* remove calls to pushInstalledFile in InstallArtifact. See ziglang#14943
* rewrite InstallArtifact to not incorrectly observe whether a Compile
  step has any generated outputs. InstallArtifact is meant to trigger
  output generation.
* fix child process evaluation code handling of `-fno-emit-bin`.
* don't store out_h_filename, out_ll_filename, etc., pointlessly. these
  are all just simple extensions appended to the root name.
* make emit_directory optional. It's possible to have nothing outputted,
  for example, if you're just type-checking.
* avoid passing -femit-foo/-fno-emit-foo when it is the default
* rename ConfigHeader.getTemplate to getOutput
* deprecate addOptionArtifact
* update the random number seed of Options step caching.
* avoid using `inline for` pointlessly
* avoid using `override_Dest_dir` pointlessly
* avoid emitting an executable pointlessly in test cases

Removes forceBuild and forceEmit. Let's consider these additions separately.
Nearly all of the usage sites were suspicious.

closes ziglang#16347

This branch was not intended to introduce new test coverage on the
emit-h feature.

See ziglang#9698

The callsites of getPath rely on the result being absolute so that they
can pass the path to a child process with the cwd set to the build root.