Do not package names representing .dsc/.deb/... files

In the case of build-dep and other commands where a file can be
passed we must make sure not to normalize the path name as that
can have odd side effects, or well, cause the operation to do
nothing.

Test for build-dep-file is adjusted to perform the vcard check
once as "vcard" and once as "VCard", thus testing that this
solves the reported bug.

We inline the std::transform() and optimize it a bit to not
write anything in the common case (package names are defined
to be lowercase, the whole transformation is just for names
that should not exist...) to counter the performance hit of
the added find() call (it's about 0.15% more instructions
than with the existing transform, but we save about 0.67%
in writes...).

Closes: #854794
(cherry picked from commit 85ee4036c68d8ecd2c973d413a17aca81380900b)
(cherry picked from commit 83e6e1a8fc942668f9a01906cb8349fb70a45b3d)

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