fix: reset rewritten state for no-decl files

[oshokin]

Summary

• reset Rewriter.rewritten at the start of RewriteFile
• prevent stale rewrite state leaking into files without declarations (e.g. doc.go)
• add regression test for sequential rewrite: failpoint file -> no-decl file

Repro

• rewrite a file with failpoint injection using one Rewriter instance
• then rewrite doc.go (package x only) with the same instance
• before fix, GetRewritten() incorrectly stays true

Test

• go test ./code -run TestRewriteFileResetRewrittenStateForNoDeclFile -count=1

[pingcap-cla-assistant]

All committers have signed the CLA.

[oshokin]

I've signed license multiple times already 😿

[kennytm]

@oshokin The commit 3d3a367 is authored by "oleg.shokin" <oleg.shokin＠myoffice.team>. If this is you, the CLA bot is unable to associate that email address with your account, and thus rejects the contribution.

You can associate that email address to your GitHub account (see https://docs.github.com/en/account-and-profile/how-tos/email-preferences/adding-an-email-address-to-your-github-account), or rewrite that commit using your GitHub email address instead.

[oshokin]

@oshokin The commit 3d3a367 is authored by "oleg.shokin" <oleg.shokin＠myoffice.team>. If this is you, the CLA bot is unable to associate that email address with your account, and thus rejects the contribution.

You can associate that email address to your GitHub account (see https://docs.github.com/en/account-and-profile/how-tos/email-preferences/adding-an-email-address-to-your-github-account), or rewrite that commit using your GitHub email address instead.

I see, thanks for explanation.
I'm gonna remake that commit better from my personal PC.