Vendoring in std and cmd

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The Go command maintains copies of external packages needed by the

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standard library in the src/vendor and src/cmd/vendor directories.

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There are two modules, std and cmd, defined in src/go.mod and

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src/cmd/go.mod. When a package outside std or cmd is imported

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by a package inside std or cmd, the import path is interpreted

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as if it had a "vendor/" prefix. For example, within "crypto/tls",

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an import of "golang.org/x/crypto/cryptobyte" resolves to

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"vendor/golang.org/x/crypto/cryptobyte". When a package with the

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same path is imported from a package outside std or cmd, it will

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be resolved normally. Consequently, a binary may be built with two

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copies of a package at different versions if the package is

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imported normally and vendored by the standard library.

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Vendored packages are internally renamed with a "vendor/" prefix

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to preserve the invariant that all packages have distinct paths.

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This is necessary to avoid compiler and linker conflicts. Adding

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a "vendor/" prefix also maintains the invariant that standard

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library packages begin with a dotless path element.

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The module requirements of std and cmd do not influence version

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selection in other modules. They are only considered when running

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module commands like 'go get' and 'go mod vendor' from a directory

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in GOROOT/src.

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Maintaining vendor directories

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Before updating vendor directories, ensure that module mode is enabled.

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Make sure that GO111MODULE is not set in the environment, or that it is

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set to 'on' or 'auto'.

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Requirements may be added, updated, and removed with 'go get'.

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The vendor directory may be updated with 'go mod vendor'.

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A typical sequence might be:

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cd src

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go get golang.org/x/net@master

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go mod tidy

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go mod vendor

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Use caution when passing '-u' to 'go get'. The '-u' flag updates

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modules providing all transitively imported packages, not only

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the module providing the target package.

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Note that 'go mod vendor' only copies packages that are transitively

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imported by packages in the current module. If a new package is needed,

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it should be imported before running 'go mod vendor'.

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