Delay loading of link.py. NFC (#20883)

This reduces the overhead of emcc when compiling but not linking.

The following result were generated using `multitime -n 5`:

```
Baseline (clang --target=wasm32 -c test/hello_world.c --sysroot=cache/sysroot):
            Mean        Std.Dev.    Min         Median      Max
real        0.067       0.002       0.064       0.066       0.070
user        0.036       0.004       0.031       0.037       0.042
sys         0.029       0.005       0.023       0.032       0.034

Before this change (emcc -c test/hello_world.c)
            Mean        Std.Dev.    Min         Median      Max
real        0.212       0.004       0.207       0.212       0.218
user        0.159       0.009       0.148       0.155       0.171
sys         0.053       0.009       0.040       0.052       0.064

After this change (emcc -c test/hello_world.c)
            Mean        Std.Dev.    Min         Median      Max
real        0.197       0.003       0.192       0.197       0.202
user        0.130       0.009       0.113       0.133       0.136
sys         0.066       0.009       0.057       0.064       0.084
```

So the overhead of emcc here was reduced from 145ms to 130ms (an 11%
reduction).

Author: sbc100

emcc.py

@@ -44,7 +44,6 @@
 from tools.response_file import substitute_response_files
 from tools import config
 from tools import cache
-from tools import link
 from tools.settings import default_setting, user_settings, settings, MEM_SIZE_SETTINGS, COMPILE_TIME_SETTINGS
 from tools.utils import read_file, removeprefix
 
@@ -628,20 +627,24 @@ def run(args):
   if state.mode == Mode.POST_LINK_ONLY:
     if len(input_files) != 1:
       exit_with_error('--post-link requires a single input file')
+    # Delay import of link.py to avoid processing this file when only compiling
+    from tools import link
     link.run_post_link(input_files[0][1], options, state, newargs)
     return 0
 
   ## Compile source code to object files
   linker_inputs = phase_compile_inputs(options, state, newargs, input_files)
 
-  if state.mode != Mode.COMPILE_AND_LINK:
+  if state.mode == Mode.COMPILE_AND_LINK:
+    # Delay import of link.py to avoid processing this file when only compiling
+    from tools import link
+    return link.run(linker_inputs, options, state, newargs)
+  else:
     logger.debug('stopping after compile phase')
     for flag in state.link_flags:
       diagnostics.warning('unused-command-line-argument', "argument unused during compilation: '%s'" % flag[1])
     return 0
 
-  return link.run(linker_inputs, options, state, newargs)
-
 
 def normalize_boolean_setting(name, value):
   # boolean NO_X settings are aliases for X