Use fs.FS in place of zip.Reader.

Leave existing code for now for backwards compatibility.

For golang/go#47834

Change-Id: Ide39dbef18504e156e70036db67221842088011f
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Replace ZipSignature with FSSignature.

For golang/go#47834

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Rewrite fetch code to use fs.FS instead of zip.Reader.

For golang/go#47834

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Pure code motion.

For golang/go#47834

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Remove the Files method. It is not used by us,
or anyone else inside Google.

Also, unexport its replacement, the paths method, because it is also
unused outside the module so there is no need to export it.

For golang/go#47834

Change-Id: Iace20d879db753575563067f60ac45e3ea034f31
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Change the license detector so it works on the content directory of
the zip, not the root.

Preserve NewDetector's behavior because it's used elsewhere.

For golang/go#47834

Change-Id: I42ba0c95cf19dc735b08a342f48323490d21aa0c
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Obtain an fs.FS for the module's content directory
at the beginning of fetching, and use it throughout.

For golang/go#47834

Change-Id: I85684b59a50e06a043affb4d65ce6962c5d23077
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An empty module path in fetch won't happen since we made the
DirectoryModuleGetter figure out its module path.

For golang/go#47834

Change-Id: I953ad425c7e13ec4e771f4160870e5ef8f7349ff
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Move the responsibility for getting the module zip's content directory
from FetchModule to the ModuleGetters.

This will enable a ModuleGetter that reads directly from the on-disk
filesystem.

For golang/go#47834

Change-Id: I035a88680569272845ad12deb09ed94e6a37bf66
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Pure code motion.

For golang/go#47834

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For golang/go#47834

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As the culmination of the last few CLs, we no longer need to build a
zip file when reading a module from a local directory.

Now `go run ./cmd/pkgsite .` takes about 12 seconds, instead of too long
to wait for.

For golang/go#47834

Change-Id: I73bdcefe01068432de37c53e3caeebf2b161c9f6
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Add a function that returns the content directory of the stdlib
module.

Remove the part of the test that checks go.mod; it was never
executing, because there is no go.mod file in any of the repos being
tested.

For golang/go#47834

Change-Id: Idc982620f6736ec60fe9a295f0372fd272745457
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For golang/go#47834

Change-Id: I81acf4fdc51a19352bfa7fb0cf9a2c22bb42a65b
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In the module cache, some directories have .info and .mod files but no
.zip, because the go command doesn't need it. We can't display these
modules without the zip. So return NotFound for these modules, even
for Info and Mod.

Also, return derrors.NotFound any time we can't find a file.

Also, rename fsModuleGetter to fsProxyModuleGetter to make it clear
that the filesystem has to be organized like the proxy protocol.

For golang/go#47834

Change-Id: Ia7d6260abc91a6172820d9de6bf759567720a443
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