Autoclean

[bobg]

This PR adds the "autoclean" feature: output files of Files targets can be added to a registry for use by Clean.

It incidentally changes Clean from a function to a type. It also configures a few new linters and fixes some things they found.

[github-actions]

Pull Request Test Coverage Report for Build 4967234382

• 74 of 85 (87.06%) changed or added relevant lines in 7 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 73.615%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
command.go	3	5	60.0%
clean.go	36	45	80.0%

Files with Coverage Reduction	New Missed Lines	%
command.go	1	77.05%

Totals
Change from base Build 4964966395:	0.3%
Covered Lines:	1621
Relevant Lines:	2202

---

💛 - Coveralls

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR requires an increase in your module’s major version number.
If the new major version number is 2 or greater,
you must also add or update the version suffix
on the module path defined in your go.mod file.
See the Go Modules Reference for more info.

checking Clean
  func(files ...string) github.com/bobg/fab.Target went from function to non-function
    Major