Resolving reference error happens with directories of different depths

[stakme]

Hi. I found the recursive lookup will fail when there are files in directories of different depths.

reporduce: https://github.com/stakme/kin-openapi/commit/cefedea2d71ad4ce07f2739ecbaea4c4f113f9cd

> go test github.com/getkin/kin-openapi/openapi3
--- FAIL: TestLoaderReadFromURIFunc (0.00s)
    loader_read_from_uri_func_test.go:20: 
                Error Trace:    loader_read_from_uri_func_test.go:20
                Error:          Received unexpected error:
                                error resolving reference "../openapi.yml#/components/schemas/Bar": open testdata/recursiveRef/components/openapi.yml: no such file or directory
                Test:           TestLoaderReadFromURIFunc
--- FAIL: TestLoaderSupportsRecursiveReference (0.00s)
    loader_recursive_ref_test.go:13: 
                Error Trace:    loader_recursive_ref_test.go:13
                Error:          Received unexpected error:
                                error resolving reference "../openapi.yml#/components/schemas/Bar": open testdata/recursiveRef/components/openapi.yml: no such file or directory
                Test:           TestLoaderSupportsRecursiveReference
FAIL
FAIL    github.com/getkin/kin-openapi/openapi3  0.192s
FAIL

It seems that, each time it resolve the reference, it needs to update the documentPath.