Add __main__ module to package

[zippolyte]

What does this PR do?

Add __main__ module to package, so that it can be executed with python -m datadog_checks.downloader datadog-vsphere

Motivation

Needed for windows so that the integration command can use the downloader

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If PR adds a configuration option, it has been added to the configuration file.

Additional Notes

Anything else we should know when reviewing?

[codecov]

Codecov Report

Merging #3108 into master will increase coverage by 15.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #3108       +/-   ##
==========================================
+ Coverage   84.97%    100%   +15.02%     
==========================================
  Files         676       1      -675     
  Lines       34940      18    -34922     
  Branches     4188       2     -4186     
==========================================
- Hits        29689      18    -29671     
+ Misses       4032       0     -4032     
+ Partials     1219       0     -1219