Site importer: fix for issue where site import preview persists from previous import

[andfinally]

This is a duplicate of #35835. That PR has been approved, but I've been unable to merge it because of an issue with the canary e2e tests. Copying the changes into a new PR enables the tests to pass.

---

When you complete a Wix or GoDaddy import, and then start a second one, you see the preview page from the previous import instead of the form inviting you to enter the URL of the next site you want to import.

Changes proposed in this Pull Request

• This change clears the importStage state of the SiteImporterInputPane component on mounting, if the state is importable. This state should never occur on first mounting: it's only set after the importer has checked a given URL for importability.

Testing instructions

• Apply the PR and run Calypso locally, or use calypso.live.
• Starting at URL: /import/[ site ], open the Wix or GoDaddy importer and import a Wix or GoDaddy site.
• When the import is complete, click on "Export" or some other sidebar menu item, and then back on "Import" to return to the importers page. (This clears the "import finished" view.)
• Open the Wix or GoDaddy importer again. You should see the site importer form with input to enter a URL, never the site preview page.

• Complete the second import and verify that all goes as normal and there are no errors in the console.
• Do a Wix or GoDaddy import from the /start flow to verify that still works as expected.

Fixes Samus issue 35335

[matticbot]

Test live: https://calypso.live/?branch=fix/site-importer-preview-persists

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~28 bytes added 📈 [gzipped])

name    parsed_size           gzip_size
import       +153 B  (+0.1%)      +28 B  (+0.1%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.