Adopt _PACKAGE sentinel for package documentation

[kylebaron]

This PR adopts the "_PACKAGE" approach. See r-lib/roxygen2#1491

Dear maintainer,

You have file 'mrgsolve/man/mrgsolve_package.Rd' with
\docType{package}, likely intended as a package overview help file, but
without the appropriate PKGNAME-package \alias as per "Documenting
packages" in R-exts.

This seems to be the consequence of the breaking change

Using @doctype package no longer automatically adds a -package alias.
Instead document _PACKAGE to get all the defaults for package
documentation.

in roxygen2 7.0.0 (2019-11-12) having gone unnoticed, see
r-lib/roxygen2#1491.

As explained in the issue, to get the desired PKGNAME-package \alias
back, you should either change to the new approach and document the new
special sentinel

"_PACKAGE"

or manually add

@Aliases mrgsolve-package

if remaining with the old approach.

Please fix in your master sources as appropriate, and submit a fixed
version of your package within the next few months.

[kyleam]

LGTM

[kylebaron]

Thanks, @kyleam