Fold target help into the help system. (pantsbuild#11022)

Instead of it being the only help-related functionality
implemented as a standalone goal.

Also replaces `./pants goals` with `./pants help goals`,
and makes various other improvements to help, such as
not showing global option help when you just enter
`./pants help`, so that the usage message fits in a
single screen.

Addresses pantsbuild#10999.

[ci skip-rust]

[ci skip-build-wheels]

build-support/bin/ci.py

@@ -295,10 +295,10 @@ def run_check(command: List[str]) -> None:
             die(f"Failed to execute `./pants {command}`.")
 
     checks = [
-        ["goals"],
+        ["help", "goals"],
         ["list", "::"],
         ["roots"],
-        ["target-types"],
+        ["help", "targets"],
     ]
     with travis_section("SmokeTest", "Smoke testing bootstrapped Pants and repo BUILD files"):
         check_pants_pex_exists()