Cache ReadWriteBuckets based on absolute path in appprotoos.ResponseWriter

[bufdev]

While investigating the feasibility of #1123, I realized that our caching would not work for absolute vs relative paths. This should fix it, but there is a test failure.

--- FAIL: TestInsertionPointMixedPathsFail (0.29s)
    appcmdtesting.go:147:
        	Error Trace:	appcmdtesting.go:147
        	           				protoc_test.go:249
        	           				protoc_test.go:221
        	Error:      	Not equal:
        	           	expected: 1
        	           	actual  : 0
        	Test:       	TestInsertionPointMixedPathsFail
        	Messages:   	

From the comments on the test, I can't figure out what is going on and why this should be failing. @amckinney can you take a quick look?

[amckinney]

So that failure is actually valid from a protoc perspective - I discovered this back when we were refactoring all of the insertion point stuff in this PR. I left a comment on the test helper:

// testInsertionPointMixedPathsFail demonstrates that insertion points are only
// able to generate to the same output directory, even if the absolute path points
// to the same place.
func testInsertionPointMixedPathsFail(t *testing.T, runner command.Runner, receiverOut string, writerOut string) {
  ...
}

So the test above SHOULD be failing because this doesn’t actually work:

$ protoc -I proto proto/buf/alpha/image/v1/image.proto --insertion-point-receiver_out=insertion --insertion-point-writer_out=$(pwd)/insertion

With that said, we could just remove this test so that buf alpha protoc actually supports absolute paths (even though protoc doesn't). In that case, we'd actually support more features than protoc, so everything else would still be backwards compatible.

I'll follow-up here by just refactoring the test verifying that mixing the path format (absolute vs. relative) works, then leave another comment justifying it.

[bufdev]

LGTM