Add http.FileSystem migration source driver support

[fln]

This PR adds a new migration source driver capable of reading files from
any source that implements http.FileSystem interface.

Notable http.FileSystem interface implementations:

• http.Dir() - wrapper over local file-system
• github.com/shurcooL/vfsgen

Because user of this package is responsible for getting an
implementation of http.FileSystem, this driver does not support creating
instances from driver URLs.

[coveralls]

Pull Request Test Coverage Report for Build 596

• 86 of 95 (90.53%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.8%) to 51.944%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
source/errors.go	0	3	0.0%
source/httpfs/partialdriver.go	77	83	92.77%

Totals
Change from base Build 565:	0.8%
Covered Lines:	4836
Relevant Lines:	9310

---

💛 - Coveralls

[wayneashleyberry]

Looks like markbates/pkger also implements http.FileSystem, I'd definitely consider that a notable implementation as it is essentially a replacement for gobuffalo/packr.

https://blog.gobuffalo.io/introducing-pkger-static-file-embedding-in-go-1ce76dc79c65

[dhui]

Thanks for the PR!
This is a good start, but the interface for httpfs.driver should change. Namely it shouldn't implement the source.Driver interface but instead be embedded by other structs which fully implement the source.Driver interface.

[dhui]

@fln Don't force push to your branch since it makes PRs harder to review
I'll re-run any failing tests that aren't due to your changes

[fln]

Thanks @dhui for reviewing and helping to push this PR forward. We are heavy users of golang-migrate/migrate package and happy to be able to contribute.

[dhui]

Thanks for addressing my feedback! Just a few more issues.
We should split up httpfs.HTTPFS and httpfs.HTTPFSPassThrough structs.
Examples/docs should be updated accordingly and showcase embedding.

[fln]

Updated the PR to expose httpfs.Migrator for embedding into new source drivers and New() for http.FileSystem conversion to source.Driver. If you approve current package API I could tidy it up, squash commits and create new clean PR.

[dhui]

Thanks for addressing the last round of feedback!

If you approve current package API I could tidy it up, squash commits and create new clean PR.

I don't think there's a need to squash commits or open a new PR since I'll do a squash an merge via github. If there's any other tidying you can think of, please feel free to do so

[fln]

All review issues are resolved for the time being 🕺.

[dhui]

This should probably be a source.Migration and an os.File instead of a filename string. I'll fix this myself to avoid an extra review roundtrip