Fix exports of undefined/missing/conditionally defined symbols (#21217)

* Remove Base.Parallel export.
* Remove 'big_str' export from mpfr.jl
* Remove 'dense' from export in sparse.jl

Author: m-j-w

base/exports.jl

@@ -23,7 +23,7 @@ export
     Markdown,
     Threads,
     Iterators,
-    Parallel,
+    Distributed,
 
 # Types
     AbstractChannel,
@@ -1344,7 +1344,7 @@ export
     nzrange,
     nnz,
 
-# Parallel module re-exports
+# Distributed module re-exports
     @spawn,
     @spawnat,
     @fetch,

base/mpfr.jl

@@ -4,8 +4,7 @@ module MPFR
 
 export
     BigFloat,
-    setprecision,
-    big_str
+    setprecision
 
 import
     Base: (*), +, -, /, <, <=, ==, >, >=, ^, ceil, cmp, convert, copysign, div,

base/sparse/sparse.jl

@@ -27,7 +27,7 @@ import Base: @get!, acos, acosd, acot, acotd, acsch, asech, asin, asind, asinh,
     triu, vec, permute!, map, map!
 
 export AbstractSparseArray, AbstractSparseMatrix, AbstractSparseVector,
-    SparseMatrixCSC, SparseVector, blkdiag, dense, droptol!, dropzeros!, dropzeros,
+    SparseMatrixCSC, SparseVector, blkdiag, droptol!, dropzeros!, dropzeros,
     issparse, nonzeros, nzrange, rowvals, sparse, sparsevec, spdiagm, speye, spones,
     sprand, sprandn, spzeros, nnz, permute