Add information in command conflict exception message when building extension command tree

[fengzhou-msft]

Now the log will show messages like this in case of conflicts:

Processing storage-blob-preview
...
Exception: Key: show already exists in storage-preview. 2 extensions cannot have the same command!

Now we know that storage-blob-preview and storage-preview have the same command ending with show. By searching the log message, we are able to find the duplicate command is az storage account management-policy show.

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This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/fengzhou-msft/azure-cli-extensions.git@ext_cmd_tree_update#subdirectory=src/$EXT&egg=$EXT"