bp2build: codegen singleton lists on one line.

This CL refactors the bp2build code generator to pretty print lists with
a single element on one line, instead of taking up three lines, which
can make BUILD files unnecessarily long. A single line singleton list is
also more commonly used in BUILD files.

Test: TH
Change-Id: Ic9e44741bbb070c8f45925466b9ccdd0608498b2

bp2build/sh_conversion_test.go

@@ -74,9 +74,7 @@ func TestShBinaryBp2Build(t *testing.T) {
 }`,
 			expectedBazelTargets: []string{`sh_binary(
     name = "foo",
-    srcs = [
-        "foo.sh",
-    ],
+    srcs = ["foo.sh"],
 )`},
 		},
 	}