Remove a few cases of underscore escaping

[wilzbach]

A start at removing the no longer needed underscore escaping from Phobos.
This was done partially by a script to get an initial set of replacements:

import std.experimental.all;
void main(string[] args)
{
    foreach (file; dirEntries(".", "*.d", SpanMode.depth))
    {
        auto text = file.readText;
        auto packageName = file.baseName.stripExtension;
        auto moduleName = file.dirName.baseName;
        foreach (rep; [packageName, moduleName])
            text = text.replace("_"~rep, rep);
        text.toFile(file);
    }
}

And then manually adding the replacements to the git index.
I might have missed a few instances (or skipped the ones which were a bit more complicated),
but we can easily find them later with the DDOC_AUTO_PSYMBOL_SUPPRESS macro).

See also:

• Start to opt-out of Ddoc's automatic keyword highlighting dlang.org#2307
• https://dlang.org/changelog/2.079.0.html#fix18361

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#6542"

[JackStouffer]

Seems simple enough

[schveiguy]

I was starting to look through all these, but there's too many! Luckily we have the doc preview, I can see only a few html files changed, and all the changes are fixes (removing unintentional underscores).