internal/imports: add a benchmark for the initial mod cache scan

In CL 508506, we missed a performance regression because there were no
benchmarks for goimports' initial scan: the only scan benchmark related
to re-scanning, which doesn't recursively descend into all module cache
directories.

Add a benchmark for the initial scan.

For golang/go#44863

Change-Id: I94ec8b2b0df468fd06a1316f24c00d5f47c2f5c9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/561436
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: findleyr

internal/imports/mod_test.go

@@ -1289,7 +1289,7 @@ import (
 	}
 }
 
-func BenchmarkScanModCache(b *testing.B) {
+func BenchmarkModuleResolver_RescanModCache(b *testing.B) {
 	env := &ProcessEnv{
 		GocmdRunner: &gocommand.Runner{},
 		// Uncomment for verbose logging (too verbose to enable by default).
@@ -1309,3 +1309,17 @@ func BenchmarkScanModCache(b *testing.B) {
 		resolver.(*ModuleResolver).ClearForNewScan()
 	}
 }
+
+func BenchmarkModuleResolver_InitialScan(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		env := &ProcessEnv{
+			GocmdRunner: &gocommand.Runner{},
+		}
+		exclude := []gopathwalk.RootType{gopathwalk.RootGOROOT}
+		resolver, err := env.GetResolver()
+		if err != nil {
+			b.Fatal(err)
+		}
+		scanToSlice(resolver, exclude)
+	}
+}