build system: follow-up enhancements regarding LazyPath

* introduce LazyPath.cwd_relative variant and use it for --zig-lib-dir. closes #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 #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.

lib/std/Build/Step/WriteFile.zig

@@ -28,7 +28,8 @@ pub const File = struct {
     sub_path: []const u8,
     contents: Contents,
 
-    pub const getFileSource = getPath; // DEPRECATED, use getPath
+    /// deprecated: use `getPath`
+    pub const getFileSource = getPath;
 
     pub fn getPath(self: *File) std.Build.LazyPath {
         return .{ .generated = &self.generated_file };