[CI] add prechek logic in job LintModifiedExtension

[haroldrandom]

Fix #1028, after approved, I will rename the job to LintModifiedExtension

Scenario 1: User modify other files, instead of source code and src/index.json. CI should pass (Please see the build's output of this PR).

Scenario 2: User modify src/index.json only and metadata item refer to extension not
inside our repo (#1333), CI will pass and job linter will skip and do nothing.

Scenario 3: User modify both src/index.json and extension source code (#1336), CI shall fail with a warning message.

Scenario 4: User modify src/index.json only and metadata item refer to extension inside our repo (#1337). CI will pass and job linter will skip and do nothing.

Scenario 5: User modify extension source code only (#1338), and CI job "CLI Linter on Modified Extensions (New)" should build and linter on modified extensions.

Scenario 6: User delete some files of source code. (#1357),
and CI job "CLI Linter on Modified Extensions (New)" should skip to build with output

----------------------------------------------------------------------------------------------------
no extension source code modified, no extension needs to be linter
----------------------------------------------------------------------------------------------------

the extension and pass. But other CI job will fail as missing some necessary files

Scenario 7: User delete the whole extension source (#1358), and CI job "CLI Linter on Modified Extensions (New)" should skip to do nothing and pass.

---

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.

[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/$EXT "git+https://github.com/Azure/azure-cli-extensions.git@release-pipeline-prechecks#subdirectory=src/$EXT&egg=$EXT"

[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/$EXT "git+https://github.com/Azure/azure-cli-extensions.git@release-pipeline-prechecks#subdirectory=src/$EXT&egg=$EXT"

[haroldrandom]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[yonzhan]

add to S166