dot in package names bites Rcpp Atttributes

[eddelbuettel]

As reported by @deepayan (in reference to Modules but breaks in even simpler cases).

Example:

edd@don:/tmp$ R -e 'Rcpp::Rcpp.package.skeleton("with.dot")'
edd@don:/tmp$ R CMD build with.dot 
edd@don:/tmp$ R CMD check with.dot_1.0.tar.gz 

which ends in

edd@don:/tmp$ cat /tmp/with.dot.Rcheck/00install.out
* installing *source* package ‘with.dot’ ...
** libs
ccache g++ -I/usr/share/R/include -DNDEBUG   -I"/usr/local/lib/R/site-library/Rcpp/include"   -fpic  -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -g  -O3 -Wall -pipe -Wno-unused -pedantic -c RcppExports.cpp -o RcppExports.o
RcppExports.cpp:10:21: error: expected initializer before ‘.’ token
 RcppExport SEXP with.dot_rcpp_hello_world() {
                     ^
/usr/lib/R/etc/Makeconf:141: recipe for target 'RcppExports.o' failed
make: *** [RcppExports.o] Error 1
ERROR: compilation failed for package ‘with.dot’
* removing ‘/tmp/with.dot.Rcheck/with.dot’

#403 is similar but just for function names. We need a similar s/\./_/ here.