sdks:java:io:google-cloud-platform:javadoc generating "cannot find symbol" warnings

Somehow `:sdks:java:io:google-cloud-platform:javadoc` generates strange "cannot find symbol" errors. Because it's javadoc task, it ended up warnings:

```

> Task :sdks:java:io:google-cloud-platform:javadoc
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java:912:
error: cannot find symbol
extends PTransform, SpannerWriteResult>
{
^
symbol: class PTransform
location: class SpannerIO
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java:912:
error: cannot find symbol
extends PTransform, SpannerWriteResult>
{
^
symbol: class PCollection
location: class SpannerIO
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java:700:
error: cannot find symbol
public abstract static class TypedRead extends PTransform>
{
^
symbol: class PTransform
location:
class BigQueryIO
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java:700:
error: cannot find symbol
public abstract static class TypedRead extends PTransform>
{
^
symbol: class PBegin
location:
class BigQueryIO
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java:700:
error: cannot find symbol
public abstract static class TypedRead extends PTransform>
{
^
symbol: class PCollection

location: class BigQueryIO
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java:452:
warning - Tag @link: can't find withQueryPriority(
TypedRead.QueryPriority) in org.apache.beam.sdk.io.gcp.bigquery.BigQueryIO.TypedRead
/usr/local/google/home/suztomo/beam6/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/testing/FakeBigQueryServices.java:80:
warning - Tag @link: can't find FakeBigQueryServerStream in org.apache.beam.sdk.io.gcp.testing.FakeBigQueryServices.FakeBigQueryServerStream
7
warnings

```

I guess the class path of javadoc and compileJava differs somehow. Why?

Imported from Jira [BEAM-9086](https://issues.apache.org/jira/browse/BEAM-9086). Original Jira may contain additional context.
Reported by: suztomo.