* Import trace_v1 and trace_v2 generated by artman

Command log:

```
set DEST ~/workspace/google-cloud-python/trace
set GOOGLEAPIS_REV origin/master

cd /tmp
git clone https://github.com/googleapis/googleapis.git
cd googleapis
git reset --hard $GOOGLEAPIS_REV
artman --config google/devtools/cloudtrace/artman_cloudtrace_v1.yaml generate python_gapic
artman --config google/devtools/cloudtrace/artman_cloudtrace_v2.yaml generate python_gapic

set SRC ./artman-genfiles/python/trace-v1/
set IMPORT_PKG trace_v1
cp -r $SRC/docs $DEST
cp -r $SRC/google/cloud/$IMPORT_PKG $DEST/google/cloud/
mkdir -p $DEST/tests/unit/gapic
cp -r $SRC/tests/unit/gapic $DEST/tests/unit
mkdir -p $DEST/tests/system/gapic
cp -r $SRC/tests/system/gapic $DEST/tests/system

set SRC ./artman-genfiles/python/trace-v2/
set IMPORT_PKG trace_v2
cp -r $SRC/docs $DEST
cp -r $SRC/google/cloud/$IMPORT_PKG $DEST/google/cloud/
mkdir -p $DEST/tests/unit/gapic
cp -r $SRC/tests/unit/gapic $DEST/tests/unit
mkdir -p $DEST/tests/system/gapic
cp -r $SRC/tests/system/gapic $DEST/tests/system
```

* Fix bug in autogen: invalid import paths in proto files

* Remove references to gax

* Trace: Fix tests after replacing gax with gapic