Use selective top-level module imports in std.{functional,getopt,math,net,outbuffer}

[wilzbach]

Doing this in multiple parts.

See also: #7024

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + phobos#7026"

[thewilsonator]

Please merge these when the auto merge queue is not so full.

[thewilsonator]

std/net/isemail.d(246): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(364): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(1817): Error: no property `back` for type `EmailPart[]`
std/net/isemail.d(1818): Error: no property `popBack` for type `EmailPart[]`
std/net/isemail.d(465): Error: template instance `std.net.isemail.pop!(EmailPart[])` error instantiating
std/net/isemail.d(776):        instantiated from here: `isEmail!char`
std/net/isemail.d(490): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(543): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(558): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(626): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(743): Error: no property `front` for type `const(char)[]`
std/net/isemail.d(751): Error: no property `popFront` for type `EmailStatusCode[]`
std/net/isemail.d(246): Error: no property `front` for type `const(wchar)[]`