Improve performance of SnapshotSubset

[Eric-Arellano]

As first discovered in #9702 (comment), calling await Get[Snapshot](SnapshotSubset(original_snapshot.digest, PathGlobs(original_snapshot.files) with 10k files in the snapshot takes 35 seconds.

To reproduce, use this diff and run ./v2 test src/python/pants/core/util_rules/strip_source_roots_test.py:

diff --git a/src/python/pants/core/util_rules/strip_source_roots_test.py b/src/python/pants/core/util_rules/strip_source_roots_test.py
index 77d4d28eb..fbd72b0e0 100644
--- a/src/python/pants/core/util_rules/strip_source_roots_test.py
+++ b/src/python/pants/core/util_rules/strip_source_roots_test.py
@@ -13,8 +13,9 @@ from pants.core.util_rules.strip_source_roots import (
 )
 from pants.core.util_rules.strip_source_roots import rules as strip_source_root_rules
 from pants.engine.addresses import Address
-from pants.engine.fs import EMPTY_SNAPSHOT
+from pants.engine.fs import EMPTY_SNAPSHOT, PathGlobs, Snapshot, SnapshotSubset
 from pants.engine.internals.scheduler import ExecutionError
+from pants.engine.rules import RootRule
 from pants.engine.selectors import Params
 from pants.engine.target import Sources as SourcesField
 from pants.testutil.option.util import create_options_bootstrapper
@@ -24,7 +25,7 @@ from pants.testutil.test_base import TestBase
 class StripSourceRootsTest(TestBase):
     @classmethod
     def rules(cls):
-        return (*super().rules(), *strip_source_root_rules())
+        return (*super().rules(), *strip_source_root_rules(), RootRule(SnapshotSubset))
 
     def get_stripped_files(
         self,
@@ -64,6 +65,15 @@ class StripSourceRootsTest(TestBase):
             "project_test/example.py"
         ]
 
+        import time
+
+        input_snapshot = self.make_snapshot_of_empty_files(f"f{n}.txt" for n in range(10_000))
+        start = time.time()
+        self.request_single_product(
+            Snapshot, SnapshotSubset(input_snapshot.digest, PathGlobs(input_snapshot.files))
+        )
+        raise ValueError(round(time.time() - start, 3))
+
         # Unrecognized source root
         unrecognized_source_root = "no-source-root/example.txt"
         assert get_stripped_files_for_snapshot([unrecognized_source_root]) == ["example.txt"]

[Eric-Arellano]

Thanks @cosmicexplorer for fixing this!!