Precompile python sources

This signifigantly improves the startup time of soong-built
python binaries. For example, running
`m apexer && time out/host/linux-x86/bin/apexer` gives
0.734s before this cl, and 0.094s after.

Fixes: 259718110
Test: Presubmits
Change-Id: Ib19e83e2c60c39a849525be117279c318de3afa7

python/builder.go

@@ -70,6 +70,17 @@ var (
 			CommandDeps: []string{"$mergeParCmd"},
 		},
 		"srcsZips", "launcher")
+
+	precompile = pctx.AndroidStaticRule("precompilePython", blueprint.RuleParams{
+		Command: `LD_LIBRARY_PATH="$ldLibraryPath" ` +
+			`PYTHONPATH=$stdlibZip/internal/stdlib ` +
+			`$launcher build/soong/python/scripts/precompile_python.py $in $out`,
+		CommandDeps: []string{
+			"$stdlibZip",
+			"$launcher",
+			"build/soong/python/scripts/precompile_python.py",
+		},
+	}, "stdlibZip", "launcher", "ldLibraryPath")
 )
 
 func init() {